Abstract.200 (ig of synthetic thyrotropin releasing hormone (TRH) was administered intravenously to six normal men and six normal women on two occasions. A positive correlation between the basal TSH level and maximal serum TSH after TRH was found. The TSH responses in the mid‐follicular and mid‐luteal phase were similar. There was no difference in the response between the men and the women investigated. The variability was considerable, the intra‐individual coefficient of variation of the TSH response being 30 per cent when expressed as per cent increase above the basal