Two groups of rats, one with surgically induced chronic renal failure and a sham-operated group were used. 10 weeks after surgery the animals were individually mated for 15 days according to 4 different schemes: uremic couple, control couple, a couple with a uremic male, and a couple with a uremic female. A second mating was allowed 18 weeks after surgery. There the uremic rats and nonfertile controls were matched against fertile controls. The experimental group showed a higher percentage of nonfertile rats, their offspring had fewer newborns and the uremic mothers had litters that weighed less at birth. In uremic males low plasma testosterone levels were detected. The analysis of plasma luteinizing hormone in the female and the delayed fertilization of uremic mothers suggest the presence of irregularities in the estrous cycle of uremic females.