The effect of 1,25-dihydroxy-vitamin D3 (1,25-(OH)2-D3) on the TRH induced TSH release was investigated. Wistar rats were injected with 1,25-(OH)2-D3 (0.05 microgram/kg/day) for three days, and TRH was injected iv on the third day. Blood was drawn every 10 min during the following 40 min, and TSH was determined. The TSH release was significantly higher in rats treated with 1,25-(OH)2-D3 than in controls. The rats treated with 1,25-(OH)2-D3 were hypercalcaemic and thus, in order to find out if the effect was mediated through hypercalcaemia rats treated as above, were infused with EDTA (30 mg/kg/100 min) starting 60 min before the TRH test. This treatment made the rats normocalcaemic, and the significant increase in the TSH release was still seen in the 1,25-(OH)2-D3 treated rats as compared to controls. The results thus indicate that 1,25-(OH)2-D3 enhances the TRH induced TSH release and that the effect is not mediated through an increase in the serum calcium concentration at the time of the TRH test. In order to find out if the effect could be mediated by changes in intracellular calcium the rats were treated with the calcium antagonist verapamil (25 mg/kg/day) and the adrenergic blocker propranolol (5 mg/kg/day) alone or together with 1,25-(OH)2-D3. In rats treated with verapamil or propranolol alone or 1,25-(OH)2-D3 + propranolol, no effect was observed on the TRH induced TSH release. Verapamil + 1,25-(OH)2-D3 significantly increased the TSH release as compared to both controls and rats treated with 1,25-(OH)2-D3 alone.(ABSTRACT TRUNCATED AT 250 WORDS)