We used very sensitive and specific monoclonal-antibody sandwich assays for human chorionic gonadotropin (hCG) and human luteinizing hormone (hLH) to measure both hormones in serum samples from normal men and women. When single serum samples from 92 men were studied, 73 percent had detectable hCG. In normal men, the amount of detectable hCG averaged 8.9 pg per milliliter, with a range of less than 3.0 to 160 pg per milliliter (biologic potency = 13,450 IU per milligram). In postmenopausal women the hCG level averaged 111 pg per milliliter and ranged from 32 to 510. In women of reproductive age the hCG level varied with the menstrual cycle. Gonadotropin-releasing hormone administered to 10 normal men increased both hCG and hLH. When daily serum samples were studied throughout a normal menstrual cycle, hCG concentrations paralleled those of hLH; follicular-phase concentrations were higher than those of the luteal phase, and there was a midcycle ovulatory peak of hCG coincident with the hLH peak. When hCG was measured every 10 minutes for six hours in eight postmenopausal women, distinct pulses were detected in parallel with those of hLH: hLH pulsed at a mean (+/- SEM) frequency of 0.56 +/- 0.08 pulses per hour; hCG pulsed at 0.54 +/- 0.07 pulses per hour. The mean pulse durations were 89 +/- 22 and 56 +/- 20 minutes for hLH and hCG, respectively. We conclude that hCG is produced in a pulsatile fashion, probably by the pituitary, in all normal adults.