Studies of normal human subjects have shown that prolactin (PRL) as well as thyrotropin (TSH) are secreted in pulsatile fashion. This study was designed to investigate whether or not these two hormones are secreted synchronously. Ten postmenopausal women who had had amenorrhea for at least 3 years and 2 women who had been ovariectomized 3 and 5 years before and were between 35 approximately 60 yr of age were studied. Blood samples were obtained between 1500 approximately 2000 h at 15 min intervals. Distinct pulsatile fluctuation of plasma TSH concentration was present in all subjects. However, only 31% of these pulses were observed to coincide with PRL pulses. The mean (+/- SD) increment of TSH (nadir to peak) was 1.2 +/- 0.5 mu u/ml. The mean interval between pulses of TSH was 73 min. In contrast, regular episodic fluctuation of plasma PRL concentration were present in only 5 out of 12 women, and 48% of these pulses were observed to coincide with TSH pulses. These results indicate that pulses of TSH and PRL are generated independently. Since thyrotropin-releasing hormone (TRH) may be a direct generator of TSH pulse and a potent stimulator of PRL secretion, our results may suggest that physiological concentration of TRH in portal blood which is sufficient to elicit TSH pulse may be too low to elicit PRL pulse.