The aim of the present study was to assess the effects of topically applied triiodothyronine (T3) and thyroxine (T4) on the arterioles of hamster cheek pouch microcirculation in vivo. Microvessels were visualized using a fluorescent microscopy technique. Topical application of T3(3.08, 30.8, 61.5, 307, 615, and 6,150 nM/l) consistently induced dose-dependent dilation of arterioles within 2.0 ± 0.5 min of administration. The application of T4(150, 257, 514, and 5,140 nM/l) caused different dose-dependent effects: dilation at the three lower doses within 16 ± 2 min and rhythmic diameter changes at the highest dose. Aging of hamsters did not alter the arteriolar responses to T3and T4. T3-induced dilation was countered by the inhibition of nitric oxide synthase with NG-nitro-l-arginine-methyl ester or NG-nitro-l-arginine. Iopanoic acid (IPA), which inhibits types I and II 5′-deiodinase, abolished the dilation elicited by 514 nM T4but did not affect T3-dependent dilation. 6-Propyl-2-thiouracil (PTU), which inhibits type I 5′-deiodinase only, did not affect the dilation induced by T4. IPA and PTU did not impair arteriolar dilation induced by acetylcholine or sodium nitroprusside. These results indicate that T3induces arteriolar dilation, likely through nitric oxide release. The local conversion of T4to T3appears to be crucial for the dilation induced by T4.