In order to investigate the role of serotonin in the regulation of thyrotropin (TSH) secretion, control and propylthiouracil (PTU)-treated male Wistar rats weighing approximately 250 g were subjected to ip injections of methysergide (MET, 10 micrograms/100 g body weight), a serotonergic receptor blocker, and killed 60 min later by decapitation. Serum and pituitary concentrations of TSH were measured by radioimmunoassay. An addition, the pituitary release of TSH was estimated in an in vitro system in which pituitary glands were incubated with hypothalamic extracts. MET treatment led to a decrease in pituitary (94.12 +/- 18.55 vs 199.30 +/- 31.47 micrograms/mg, N = 20), and serum (1.95 +/- 0.92 vs 4.26 +/- 1.40 ng/ml, N = 20) TSH concentration (P < 0.001) and also to a decreased in vitro pituitary response to control hypothalamic extracts (55 +/- 8 vs 78 +/- 7%, N = 5, P < 0.005). In addition, hypothalamic extracts of MET-treated rats significantly facilitated in vitro pituitary TSH secretion, suggesting an enhanced hypothalamic thyrotropin releasing hormone (TRH) activity (347 +/- 62 vs 78 +/- 7%, N = 5, P < 0.001). These results suggest that serotonin participates in the physiological control of TRH/TSH secretion, probably by increasing TRH production/secretion, and/or by facilitating the pituitary TSH response to TRH.