Adult, female rhesus monkeys ingested lead daily for three 1-year exposures over a 5-year period followed by chronic lead intake for 3 additional, consecutive years. Lead was administered as lead acetate in the drinking water at levels producing average circulating concentrations of lead between 44 and 89 micrograms/100 ml and zinc protoporphyrin concentrations between 87 and 105 micrograms/100 ml. The monkeys were without overt signs of lead toxicity at all times; i.e., appetite, body weight, and hematocrit levels were normal. Menstrual cycles were altered by lead. During the last 2 years of exposure, the lead-treated monkeys had less frequent cycles, longer and more variable intercycle intervals, and fewer days of vaginal bleeding than controls. These differences were not due to seasonal artifacts or differences in exposure to environmental influences such as light or diet. Further, historical data indicated that cycles were normal for the treated monkeys prior to exposure to lead and that three of four cycle characteristics remained normal during the first lead exposures. Duration of vaginal bleeding was initially attenuated by lead but returned to normal when lead intake was discontinued between exposures. In contrast, the effects of lead observed during the later years of exposure remained apparent 1 year following termination of lead ingestion. These results suggest that persistent alterations of the menstrual cycle and perhaps premature menopause resulted from prolonged exposure to lead at levels which produced no obvious signs or symptoms of toxicosis.