The traditional model for radiation risk is based on the assumption that ionizing radiation risk is directly proportional to the absorbed dose without a threshold, which is well known as the linear no-threshold (LNT) hypothesis, and which forms the fundamental basis for the prediction on health risk caused by ionizing radiation exposures. Under the LNT hypothesis, evidence from epidemiological studies of relatively high dose (greater than 100 mSv) of ionizing radiation has been used to estimate the dose response both from high-dose and low-dose ionizing radiation. As such, there has been serious concern regarding the feasibility of predicting the dose response in the low-dose regime (smaller than 100 mSv) by extrapolating the results from the high-dose regime. Recent research findings in the low-dose regime demonstrated the occurrence of non-traditional radiation effects, including the radiation-induced bystander effect (RIBE), radioadaptive response (RAR), radiation-induced rescue effect (RIRE) and hormesis. As a consequence, the dose response in this regime can no longer be described by linear relationship which is the fundamental assumption of the LNT hypothesis. Better understanding on these non-traditional radiation effects is highly relevant for the estimation of the health risks from medical, environmental as well as occupational radiation exposures. Most of the studies on non-traditional radiation effects involved radiation with low linear energy transfer (LET), such as X-ray or gamma ray. However, studies on non-traditional effects induced by high-LET radiations such as alpha particles are also pertinent in providing estimates on the risk from exposures to, e.g., radon and contamination from nuclear accidents. The present thesis is devoted to studying non-traditional effects induced by high-LET radiations. Chapter 1 gives the introduction and literature review. Chapter 2 describes a dosimetric study on alpha-particle induced radioadaptive response (RAR) for zebrafish embryos. The alpha-particle priming dose which could effectively induce in vivo RAR in the embryos was found to be 0.032 to 0.32 mGy, through the apoptotic signals scored in the embryos at 24 h post fertilization. Specially fabricated thin polyallyldiglycol carbonate (PADC) films were used as support substrates for the dechorionated embryos. During irradiation, alpha particles passed through the PADC films to hit the zebrafish embryos, and the alpha-particle tracks were revealed in the films through chemical etching. The dose range for successful induction of RAR was discussed with the possible involvement of RIBE (which will be discussed in detail in Chapter 3). Chapter 3 focuses on the radiation-induced bystander effect (RIBE) in vivo between embryos of zebrafish. Investigation on the potential chemicals that could protect the cells from damages caused by RIBE was carried out. In the present work, the influence of a low concentration of exogenous carbon monoxide (CO) liberated from tricarbonylchloro(glycinato)ruthenium (II) (CORM-3) on the RIBE in vivo between embryos of the zebrafish was studied. A significant attenuation of apoptosis on the bystander embryos induced by RIBE in a CO concentration dependent manner was observed. Chapter 4 reports data demonstrating that the bystander unirradiated zebrafish embryos can release a feedback signal back to the irradiated embryos. This phenomenon was referred to as the "rescue effect" where the bystander unirradiated embryos successfully helped the irradiated embryos mitigate the radiation induced DNA damages. The results showed that the number of apoptotic signals in the irradiated embryos was smaller when they were partnered with bystander unirradiated embryos in the same medium. The results also showed significantly fewer apoptotic signals in the irradiated embryos when the population of bystander embryos increased from 10 to 30, while keeping the population of irradiated embryos at 10. These suggest that the stress communicated between the unirradiated zebrafish embryos and the irradiated embryos sharing the same medium will help "rescue" the irradiated embryos, and that the strength of the rescue effect depends on the number of rescuing bystander unirradiated embryos. Chapter 5 describes the radiation-induced hormetic effect in zebrafish embryos. Hormesis describes the phenomenon that low doses of a stressor drop the toxic effect to below the spontaneous level. The experimental data showed that embryos of the zebrafish subjected to a low-dose alpha-particle irradiation (<2.8 mGy) could release a stress signal into the water, which could be communicated to unirradiated bystander zebrafish embryos sharing the same water medium to induce a hormetic effect in the bystander embryos. Chapter 6 gives the conclusions and presents discussion and suggestions for future studies.