Modulation of endocrine function is frequently a confounding factor in the interpretation of chronic rodent toxicology studies. Of particular interest are agents that cause deviation of thyroid hormone homeostasis and result in thyroid cancer for rodents. An endocrine challenge test (ECT), commonly used to study endocrine organ health in human and veterinary medicine, quantifies the response of the thyroid to tropic hormones. This study compared the response of Fischer (F344) and Sprague-Dawley (SD) rats to a thyrotropin-releasing hormone (TRH) ECT and a thyroid-stimulating hormone (TSH) ECT and characterized the dose-response curve. TSH, thyroxine (T4), triiodothyronine (T3), and prolactin responses were characterized for several doses of TRH over a 4-h time period. Animals were equipped with intra-atrial cannulae and were free moving at all times during blood sampling. Both strains of rats responded to intravenous TRH by releasing TSH into their blood in a dose-responsive fashion. At doses of > or = 100 ng, TSH concentrations were increased by more than 2-fold at 2 min. Concentrations reached a maximum at 15 min for doses of 100 ng/100 g body weight (bw) to 5000 ng/100g bw. The effective dose 50 (ED50) of TRH (that dose causing release of half maximal TSH concentrations) was 61 ng in F344 rats and 78 ng in SD rats. The ED75 was 173 ng and 217 ng/100 g bw, respectively. The response of T4 and T3 after TRH ECT and TSH ECT was highly variable. F344 rats responded with an increase in levels of both hormones, starting at 60 min and continuing through 240 min. In SD rats, the presence of a thyroid hormone response (T4) was present, although that of T3 was not clear. These data provide essential information for design of toxicology studies focused on the effects of toxicants and drugs on the pituitary-thyroid axis.