[en] The parent study has investigated hereditary radiation effects in progeny of the second and third generations of Wistar rats after irradiation of both grandparents with doses of 2-4 Gy. Attention was focused on the relationship between stages of gametogenesis in grandparents at the moment of radiation exposure and death of progeny in embryogenesis and early postnatal ontogenesis. Totals of 4207 mature males and females Wistar rats, 13539 offspring of the second generation (F2) and 746 offspring of the third generation (F3) were the subjects of investigation. Male and female rat grandparents (P) of 220-250 g were exposed to an external single irradiation by gamma rays to doses of 2,3 and 4 Gy at a dose rate of 0.003 Gy.s^-1 (⁶⁰Co source). The animals were mated at different times after irradiation so that different stages in gametogenesis were studied. When F1 sexual maturity was achieved, F1 males of each experimental group were mated with intact females to produce F2 descendants from the father's line and conversely F1 females use mated with intact males to produce F2 descendants from the mother's line. Embryogenesis F2 was studied after euthanasia of some females by ether on the 20th day of pregnancy. The fetuses were scored for size and mass, pathology of viscera and skeleton and the total, pre- and postimplantation death of embryos was calculated. In addition development of young rats was observed for 30 days after the birth. The numbers surviving on the 1st and 30th days after the birth were calculated, giving the death rate for this period of time. It was stated: for F2; radiation effects depend upon a dose stage of gametogenesis of both P at the time of radiation exposure; F2 death in embryogenesis occurred only after irradiation of P with a dose of 4 Gy; ossification disorder of the skeleton in F2 embryogenesis was found in all experimental groups; high F2 death is observed after birth which depends much more on the stage of gametogenesis P than radiation dose; maximum death F2 occurs after irradiation of spermatogonia and spermatocytes P; death F2 from males F1 and non-irradiated females is higher than from females F1 and non-irradiated males; based on these observed radiation effects in F2 one may draw up a list of grandparental germ cells, irradiated at different stages of maturation, in the following decreasing order of effect: matured/maturing oocytes+spermatocytes/spermatogonium/spermatids/spermatozoids; for F3: radiation exposure to males F2 (from both irradiated P) and their pairing with non-irradiated females results in more significant radiation effects in F3 than the same radiation exposure to F2 from both non-irradiated parents (control). Taking into account the universal nature of the biological effect of ionizing radiation on mammals, it is reasonable to assume that the same mechanisms as shown here in rats would also operate in humans. It is, however, not possible to predict how the actual numerical results presented here would apply human species. (author)