In the present study the influence of dexamethasone treatment of rats on the basal values of thyrotrophin (TSH) and prolactin (PRL) and the response of both hormones to thyrotrophin releasing hormone (TRH) has been investigated. Male rats were given 100 mug of dexamethasone/rat for 8 days at the same time of day. Four hours after the last administration of dexamethasone 200 ng or 100 mug of TRH/rat was injected ip. Blood was collected 10 min later by decapitation. TSH and PRL were estimated by radioimmunoassay (RIA) using the NIAMD kits. The basal and TRH stimulated values of PRL in plasma were significantly lower in dexamethasone treated rats than in controls (P less than 0.01). The basal TSH levels in the treated animals were also lowered (P less than 0.05). After 200 ng TRH/rat the increase in TSH was not as high in both groups than after the administration of 100 mug/rat. There was no significant difference between the response of TSH to TRH in the dexamethasone treated and the control rats. The different effects of dexamethasone on PRL and TSH release after TRH may give a further insight into the different regulating mechanisms of both hormones in rats.