Male and female B6D2F1 (C57BL/6J x DBA/2J)F1 and B6CBAF1 (C57BL/6J x CBA/CaHT6J)F1 mice were injected subcutaneously 3 times a week with L-deprenyl (0.25 mg/kg) starting at mean ages of 26 months and 18.5 months, respectively. Life spans of aging mice were increased 6-9% by the drug. While none of the life span effects were significant for a single genotype and gender, life spans were significantly longer in L-deprenyl-treated animals (p = .011) when all data were combined. L-deprenyl-injected mice consumed about the same amounts of food as controls: L-deprenyl 3.1 g/day, control 3.3 g/day, after 7 months of treatment. There were no significant effects of L-deprenyl on measures of changes with age in the following biological systems: activity, excitement, red blood cell mass, collagen denaturation rate, and wound healing rate. L-deprenyl-treated B6CBAF1 males and females were significantly heavier than controls after 4-6 months of treatment. To measure fecundity, B6CBAF1 males at an average age of 750 days were each caged with two young B6 females; 10 of 17 L-deprenyl-injected males sired an average of 31.3 pups per male, while 14 of 24 controls sired 82.1 pups per male.