Radioadaptive response (RAR) refers to a biological response whereby an exposure of cells or animals to a low dose of radiation induces mechanisms that protect the cells or animals against the detrimental effects of a subsequent radiation exposure. The discovery of RAR has raised queries to the traditional methodology in estimating the radiation risk in the low-dose region, which is usually achieved through extrapolation from those determined at high doses. Since then, this important defense mechanism has stimulated a series of in vitro studies. However, relatively little has been studied using in vivo models. The present study focused on the study of RAR induced by high-linear-energy-transfer (LET) radiations which provided the priming adaptive dose in zebrafish embryos in vivo. The objectives of this study were to first design the experimental setup for investigating RAR in vivo, then to investigate the RAR induced through bystander effects, and finally to study the potential effect on RAR from the effectiveness in generating DNA double strand breaks (DSBs) during the priming exposure. Two types of high-LET radiations with different effectiveness in causing DNA DSBs were used, namely, (broad-beam) alpha particles from a radioactive source and (microbeam) protons from an accelerator. The studies on the biological effects of both alpha particles and protons have real-life applications. Alpha particles are ubiquitous in our natural environment, e.g., they are constantly emitted by the naturally occurring radon gas and its progeny, which contribute to the largest natural radiation dose to human beings. On the other hand, protons are dominant in solar-radiation spectra, so a study on the biological effects from exposure to protons is important for human interplanetary missions. Chapter 1 gives the introduction and literature review. Chapter 2 describes the design of the experimental setup and the associated procedures for alpha-particle-induced RAR. Dechorionated zebrafish embryos at 5 hours post fertilization (hpf) were irradiated by alpha particles using a planar 241Am source first to provide a priming exposure and subsequently to provide a challenging exposure, and the effect of the time interval between the priming and challenging exposures, chosen as 1, 2, 4 and 5 hours, was examined. Quantification of apoptotic signals was achieved using acridine orange staining. When compared with the control cases, the amount of apoptotic signals decreased significantly in the adapted embryos for the interval time of 5 h between the priming and challenging exposures while no significant decreases in the amount of apoptotic signals for the time intervals of 1, 2 and 4 h. The results gave evidence to support the existence of RAR induced by alpha particles with a 5 h interval between the priming and challenging exposures. The importance of DNA repair in the induction of RAR was also demonstrated by the absence of RAR if the priming dose was introduced to the embryos when their repair system had not yet started operative. Chapter 3 describes the study on RAR induced by the embryo-to-embryo bystander effect. In contrast to the work described in chapter 2, the priming exposure here for the studied embryos (adapted group) was established through sharing the medium with another group of embryos that had been irradiated by alpha particles. Other experimental conditions and the experimental endpoint described in the previous chapter were employed. Again, the zebrafish embryos were irradiated by alpha particles to provide a challenging exposure 5 hours after the priming exposure. In four out of six experiments, the number of apoptotic signals was significantly reduced in the adapted group. Chapter 4 explains the procedures of and summarizes the results from the experiments on the use of microbeam protons with an energy of 3.4 MeV to provide the priming exposure to the 5 hpf embryos. The microbeam facility enjoyed advantages of being able to deliver a selected number of protons to chosen positions. To exploit these advantages, 5, 10 or 20 protons were delivered to 10 chosen positions on each embryo. The subsequent challenging exposure was delivered through whole embryo irradiation with 2 Gy of X-ray photons, with a 5 h interval between the priming and challenging exposures. Terminal dUTP transferase-mediated nick end-labeling (TUNEL) assay was used as the end point to reveal the apoptotic events. The results showed a significant decrease in the number of apoptotic signals in the adapted group with the magnitude of adaptation ranging from 14 % to 21 % for the delivery of 5 and 20 protons to each embryo, respectively, as the priming exposure. RAR could also be induced through the delivery of 10 protons to each embryo as the priming exposure, however, only one statistically significant result was obtained out of 3 sets of experiments. Chapter 5 gives the conclusion and presents discussion and suggestions for further studies.