The effect of thyroxine on the elementary process of contraction of vascular smooth muscle was tested in 38 hyperthyroid and in 38 control rats. Hyperthyroidism was induced by i.p. injections of triiodthyronine or 1-thyroxine for 2 weeks. By means of afterloaded isotonic contractions of the tetanized portal vein force velocity relations were calculated. There was a slight increase in both the extrapolated velocity of shortening at zero load [from 0.93±0.04 ML/s (control rats) to 1.03±0.04 ML/s (hyperthyroid rats;P<0.05)], and="" the="" peak="" force="" generation="" (from="" 14.8±0.4="" mn="" to="" 16.1±0.4="">P<0.005). the="" maximum="" of="" mechanical="" power="" development="" at="" a="" distinct="" extent="" of="" afterload="" was="" augmented="" from="" 1.35±0.06="">W/ML to 1.68±0.06 W/ML (P<0.0005). the="" influence="" of="" thyroxine="" on="" the="" elementary="" process="" of="" contraction="" in="" vascular="" smooth="" muscle="" is="" discussed="" in="" connection="" with="" the="" much="" stronger="" effect="" of="" hyperthyroidism="" on="" cardiac="" muscle="">