The patterns of plasma LH, FSH and prolactin concentrations were investigated in rats with a polycystic ovary condition (PCO). The condition was induced by treatment with oestradiol valerate 9 weeks before blood sampling. Serial blood samples were taken at 10-min intervals for 4 h from ten rats with PCO. All samples were assayed for LH, those from five animals for FSH and those from the remaining five animals for prolactin. In addition, five control animals with normal oestrous cycles were sampled during oestrus and the samples assayed for LH. Mean concentrations of LH, FSH and prolactin in rats with PCO were 140 ng/l, 76 micrograms/l and 7.6 micrograms/l respectively. All three hormones exhibited an episodic pattern. The mean peak amplitudes of LH, FSH and prolactin were 120 ng/l, 25 micrograms/l and 3.5 micrograms/l respectively. All three hormones exhibited a similar mean frequency of four or five episodes per 4 h. The LH and FSH patterns were closely synchronized; nearly all FSH peaks coincided with LH peaks. The prolactin pattern did not, however, correlate with that of the gonadotrophins. Despite the persistent oestrous condition of the animals with PCO, it was clear that their pattern of LH did not resemble that of cyclic animals in normal oestrus; in the normally cyclic animals in oestrus the pulse period was nearly twice as long and the pulse amplitude was more than sixfold greater than those in animals with PCO. We conclude that the unique episodic patterns of gonadotrophins are more important than mean blood concentrations of these hormones in establishing and maintaining the polycystic ovary syndrome.