The effect of serotoninotropic substances with positive and negative effects on serum concentrations of thyrotropic hormone, total triiodothyronine, and total thyroxin was studied in old male rats. Chronic treatment with L-tryptophan and citalopram reduced serum level of total thyroxin in old intact animals and serum concentration of thyrotropic hormone in old thyrectomied rats. DL-p-chlorophenylalanine increased the serum concentrations of total thyroxin in old intact rats and in thyroidectomied animals treated with triiodothyronine. Chronic methysergide therapy was associated with reduction of serum concentration of thyrotropic hormone in old intact and old thyroidectomied rats and of total triiodothyronine in old intact animals.