This study was undertaken to investigate the effect of premedicants, diazepam and atropine sulfate in ten surgical patients by determining human plasma thyroxine levels. Tetrasorb-125 was utilized as a kit which offers a method for quantitative measurement of total circulating serum thyroxine. The plasma thyroxine level before administration of premedicants was 11. 16+/-2. 45ug/100ml and at 60 minutes after administration of premedicants. it was 11. 3+/-2. 73ug/100ml so that clinical significance of the result was absent. The number of male and female patients was 3 and 7, mean age 40. 1+/-9. 82 and average body weight 56. 1+/-4. 01kg. Consequently there was no significant change in human plasma thyroxine during one hour :of premedication.