Five groups of male rhesus monkeys, Gp. I--sham-vasectomized stock diet-fed (SVS), Gp. II--vasectomized stock diet-fed (VS), Gp. III--sham-vasectomized atherogenic diet-fed (SVA), Gp. IV--vasectomized atherogenic diet-fed (VA) and Gp. V--atherogenic diet-fed vasectomized monkeys (AV) were observed for a period of 1 1/2 years. Vasectomy per se had no effect on the level of serum or tissue lipids either alone or in combination with atherogenic diet feeding. Following vasectomy the involvement of aorta by fatty lesions was significantly increased and this occurred in animals of all four experimental groups when compared with the control animals. The maximum increase was noted in group IV. Again, there was a significant increase in the frequency of aortic plaques in groups II, III and IV while it was increased in coronary arteries of all the experimental animals (Gps. II, III, IV and V). Group IV monkeys showed higher frequency of plaques as compared to group V animals. Although the plaque height was increased in all the vessels of the experimental animals, significant differences were observed only in the coronary arteries of group II monkeys and in intra-cranial cerebral vessels of group III animals. This study shows that vasectomy per se tends to increase atherosclerosis in rhesus monkeys.