Hypothalamic serotonin (5-HT; content and turnover) and monoamine oxidase (MAO) activity were measured in female catfish, Heteropneustes fossilis, after ovariectomy and supplementation with oestradiol-17 beta (OE2) in the recrudescent and quiescent phases. These factors were correlated with changes in plasma levels of OE2 and gonadotrophin. In the quiescent phase (December), neither ovariectomy nor OE2 supplementation had any significant effect on 5-HT content and MAO activity. Plasma levels of OE2 and gonadotrophin were undetectable in both control and treated fish, indicating that there was no feedback effect. In the recrudescent phase (prespawning, May), ovariectomy caused biphasic responses of MAO activity and 5-HT content. The enzyme activity decreased significantly after 2, 3, 4 and 5 weeks but increased significantly 6 weeks after ovariectomy. The 5-HT content varied in a biphasic manner with a significant increase at 2, 3 or 4 weeks and a significant decrease in week 6; there being no effect in week 5. 5-HT turnover was inhibited significantly only in week 4 after ovariectomy and did not show a biphasic pattern. In the ovariectomized groups, the OE2 level decreased significantly in a progressive manner with a maximum reduction in week 6. The plasma level of gonadotrophin showed a significant bimodal pattern of increase with the peak in week 4 after ovariectomy, indicating a strong negative feedback effect of OE2. The bimodal pattern of pituitary gonadotrophin release could be correlated with a similar pattern of increase in 5-HT content. OE2 treatment of fish which had been ovariectomized 3 weeks previously had dose-dependent effects on the enzyme; the low dose (0.1 microgram/g body weight) was stimulatory and the higher doses (0.5, 1.0 and 5.0 micrograms/g body weight) were inhibitory. The reverse was true for 5-HT content. Serotonergic turnover increased significantly only in the groups given high doses (1.0 and 5.0 micrograms/g body weight). The low dose of OE2 (0.1 micrograms/g body weight) restored the gonadotrophin and OE2 levels to those of the sham-ovariectomized vehicle-treated control group, whereas the high doses (0.5, 1.0 and 5.0 micrograms/g body weight) decreased the release of gonadotrophin in a dose-dependent manner. Our results suggest that OE2 modulates MAO activity to alter hypothalamic 5-HT in a seasonally dependent manner. The ovariectomy-induced changes in plasma levels of gonadotrophin appear to be mediated, at least partly, by the feedback action of OE2 on 5-HT metabolism.