The influence of three-day oral administration of 100 microgram 25-hydroxycholecalciferol (25-HCC) on thyrotropin (TSH), triiodothyronine (T3), calcium and phosphorus plasma levels was investigated along with changes of Qet and Qat intervals on the electrocardiographic tracing. It was found that 25-HCC in an acute trial significantly elevates the TSH level and causes a slight decline of the T3 level. Changes in the calcium and phosphorus levels were not significant. No changes of the electrocardiographic tracing were found. The results suggest that 25-HCC interferes with the process of secretion (or metabolization) of TSH (and perhaps also T3), probably via changes of the calcium ion concentration in cytosol of secretory cells.