Toxic effects of N,N-dimethylnitrosamine (DMNA) at doses of 100 or 500 micrograms l-1 on in vitro carp oocyte maturation (steroidogenesis), embryonic development and hatching of larvae (obtained as a result of artificial spawning of females kept for four seasons in normal and eutrophicated ponds) were investigated. There were no significant effects of DMNA on oocyte maturation and steroidogenesis during 24 h of incubation. The DMNA decreased the hatching of fertilized eggs derived from control females. This decrease reached a level of significance at a dose of 500 micrograms l-1. However, the effect of long-term exposure of female fish to eutrophied water was very much higher. The trend of the in vitro DMNA effect was the same, but it did not reach a statistically significant level. The results suggest that nitrosamines, through their effect on egg hatchability, may reduce fish populations along with increasing aquatic eutrophication.