It has been common to study DNA damage responses in vertebrates using cell cultures. However, such experiments cannot be used to study dynamic in vivo processes such as temporally and spatially regulated patterns of gene expression. In recent years, the zebrafish, Danio rerio, a small vertebrate from Southeast Asia, has become a preferred model for studying human disease, including carcinogenesis. The most important advantage is that the human and zebrafish genomes share considerable homology, including conservation of most DNA repair-related genes. Rapid embryonic development is another advantage in that major organ systems become evident within 48 hours postfertilization (hpf). Recently, a number of research works using the zebrafish embryo as an in vivo model to study the DNA damage response to ionizing radiation have emerged. Despite the success of using the zebrafish embryos to study the DNA damage response to ionizing radiation, only energetic photons (X-rays and gamma rays) were used. Studies using alpha particles will be of interest because alpha particles are also an ionizing radiation, and with high linear energy transfer (LET). Furthermore, alpha particles are emitted from radon and its progeny, which are ubiquitous in our natural environment, and constitute the largest natural radiation dose to human and can induce lung cancers. The objective of this study was to investigate the effect of alpha particles on zebrafish embryos. Hormetic effect and bystander effect were found in zebrafish embryos in vivo. Chapter 1 gives the introduction and the literature review. Chapter 2 describes the study of hormetic effect in zebrafish embryos in vivo. Dechorionated zebrafish embryos were irradiated at 1.5 hpf to low-dose alpha particles from an 241Am source, viz., 0.3, 0.6, 1.2, 2.4 mGy (determined using Monte Carlo simulations). At 24 hpf, these embryos were then examined for apoptotic cells through acridine orange staining. The mean number of apoptotic cells was found to decrease significantly from controls to 0.3-mGy irradiation, and then to increase almost linearly to 0.6, 1.2 and 2.4-mGy irradiation. This trend was a typical characteristic of a hormetic effect. Chapter 3 describes the study of bystander effect in zebrafish embryos in vivo. Dechorionated zebrafish embryos at 1.25 hpf were irradiated with alpha particles from an 241Am source. Thin polyallyldiglycol carbonate (PADC) films with a thickness of 16 m were used as support substrates for holding the embryos and recorded alpha-particle hit positions, and thus enabled calculation of the dose absorbed by the embryos. The irradiated embryos were subsequently incubated with naïve (unirradiated) embryos in such a way that the irradiated and naïve embryos were spatially separated but the medium was shared. Acridine orange was used to perform in vital staining to show cell deaths in the naive embryos at 24 hpf. The results gave evidence in supporting the existence of alpha-particle-induced bystander effects between zebrafish embryos in vivo, and a general positive correlation between the cell death signals in the naive embryos and the alpha-particle dose absorbed by the irradiated embryos. Chapter 4 gives the conclusions and discussion for further studies.