Microsomal enzyme inducers that increase UDP-glucuronosyltransferase (UDP-GT) activity are suspected to affect the thyroid gland by increasing the glucuronidation of T(4), which reduces serum thyroxine (T(4)). In response to reduced serum T(4), serum thyroid-stimulating hormone (TSH) increases. However, not all microsomal enzyme inducers that reduce serum T(4) produce an increase in serum TSH. We have shown that serum TSH is increased the most in rats treated with the microsomal enzyme inducers phenobarbital (PB) or pregnenolone-16alpha-carbonitrile (PCN), whereas TSH is affected less in rats treated with 3-methylcholanthrene (3MC) and Aroclor 1254 (PCB). It is unclear why serum TSH is differentially affected by various microsomal enzyme inducers. We propose that the glucuronidation of T(3) might be the reason serum TSH is increased by some microsomal enzyme inducers but not by others. Male Sprague-Dawley rats were fed either a basal diet or a diet containing PB (at 300, 600, 1200, or 2400 ppm), PCN (at 200, 400, 800, or 1600 ppm), 3MC (at 50, 100, 200, or 400 ppm), or PCB (at 25, 50, 100, or 200 ppm) for 7 days; and T(4) and T(3) UDP-GT activities were then determined. T(4) UDP-GT activity was increased in rats treated with PB (120%), PCN (250 to 400%), 3MC (400 to 600%), or PCB (300 to 430%). In contrast, T(3) UDP-GT activity was increased in rats treated with PB (90%) or PCN (120 to 200%), whereas 3MC and PCB treatments did not have an appreciable effect. In conclusion, differential effects on T(3) glucuronosyltransferase activity were found in rats treated with microsomal enzyme inducers.