Six lactating dairy cows were assigned randomly to a diet either of low fiber (30% corn silage) or of high fiber (70% corn silage). Both groups were subjected to 10 days thermoneutral (17.6 degrees C) and 10 days of environmental heat (31.2 degrees C) to determine effects of dietary fiber content and environmental heat on concentrations of thyroxine and triiodothyronine in plasma, milk, urine, and feces. During heat there were significant reductions in concentrations of thyroxine and triiodothyronine in plasma and in excretion of thyroxine and triiodothyronine in milk. Excretion of thyroxine and triiodothyronine increased in urine during heat stress, whereas concentration and excretion of thyroxine and triiodothyronine in feces decreased. Dietary fiber content had little effect upon thyroxine and triiodothyronine concentrations in plasma or excretion in milk, feces, and urine. Excretion of triiodothyronine in milk was negatively correlated with rectal temperature and positively with content in plasma. The reduction in thyroxine and triiodothyronine of plasma and the decline in excretion of thyroxine and triiodothyronine suggested that environmental heat reduced synthesis of both hormones.