In two experiments male rats were exposed to a constant magnetic field (CMF) of 0.4 T either once for 3 hours or 56 times for the same time (throughout the entire spermatogenetic cycle). During the first week after exposure they were mated with untreated females. Some of the females were sacrificed on their 21st day of gestation. The following parameters were measured: percentage of implantations and resorptions, total fetal lethality, number and weight of alive fetuses, weight of placentas and ovaries, hydration of placentas and fetuses. At birth the duration of gestation, the amount of alive and dead newborns, their weight and distribution in the litter were determined. These parameters were used to evaluate the genetic quality of spermatozoa that were involved in fertilization. The pups were observed during the first month of life. The results obtained suggest that mature spermatozoa are resistant to a single CMF exposure and that this exposure causes no mutations in the gametes which may reduce fetal viability. However, chronic exposure to CMF leads to a small and significant increase of preimplantation lethality of fetuses which may indicate a higher frequency of lethal mutations in the gametes.