The influence of longterm deprenyl treatment on the sexual performance and lifespan of male rats was studied. One hundred and thirty two rats were treated from the end of their 2nd year of life either with saline (1 ml/kg, s.c.) (n = 66) or with deprenyl (0.25 mg/kg, s.c.) (n = 66) three times a week until death. Whereas none of the two-year-old saline-treated rats displayed full scale sexual activity, this appeared in 64 out of 66 rats on deprenyl. The longest living rat in the saline-treated group lived 164 weeks. The lifespan of the group was 147.05±0.56 weeks. The shortest living animal in the (-) deprenyl-treated group lived 171 weeks and the longest living rat died during the 226th week of its life. The lifespan was 191.91 ± 2.31 weeks. This is the first instance that a well aimed medication prolonged lifespan of members of a species beyond their maximum age of death (182 weeks in the rat). A close relation between sexual activity and lifespan was detected.