Thyrotropin (TSH) secretion was assessed in three groups of healthy volunteers before and after four days administration of 1.25(OH)2 vitamin D3 (calcitriol) by the oral route -1.5 micrograms/d (group A), 3.0 micrograms/d (group B) or 3.0 micrograms/d combined with trifluoperazine (8-12 mg/d by mouth) (group C). The control trials and experiments proper were made in the same subjects within one month. The lower calcitriol dose did not change significantly the TRH-stimulated TSH levels nor the secretory reserve measured as the difference of TSH levels at the rest and TRH-stimulated levels (delta TSH) during the 20th, 30th, 40th and 60th minute following TRH administration. A larger calcitriol dose caused a significant increase of TSH values at rest and TRH-stimulated values during the 20th, 30th, 40th and 60th minute following TRH administration (p less than 0.05, p less than 0.05, p less than 0.05, p less than 0.05 and p less than 0.01, respectively) as well as delta TSH (p less than 0.05 at all time intervals). The intracellular calcium antagonist trifluoperazine interfered only with the stimulating effect of calcitriol on the TSH secretion at rest and on delta TSH during the 60th minute following TRH administration, while the TSH levels during the 20th, 30th, 40th and 60th minute after TRH administration were significantly higher, as compared with the control examination (p less than 0.01, p less than 0.01, p less than 0.05 and p less than 0.05, respectively) and also delta TSH was significantly elevated during 20th, 30th, and 40th minute after TRH administration during combined treatment (p less than 0.05, p less than 0.01, p less than 0.05, respectively). Calcitriol thus causes a dose-dependent increase of TSH secretion, probably partly also by a mechanism which is independent on the change of intracellular calcium homeostasis.