Reproductive state of animals frequently is overlooked when examining endocrine functions of the thyroid gland and adrenal cortex. This experiment was done to determine effects of reproductive state on basal and stimulated concentrations of thyroxine (total T4), 3,5,3'-triiodothyronine (total T3) and cortisol in serum of adult Beagle dogs. Five male, 5 anestrous, 5 proestrous, 5 diestrous, 5 pregnant and 5 lactating dogs were fasted for 18 h before each dog received 5 IU of thyrotropic hormone (TSH) i.v. and 2.2 IU/kg of body weight of adrenocorticotropic hormone (ACTH) i.m. Blood samples were collected via jugular cannulas or by jugular venipuncture at 60, 45, 30, 15 and 1 min before and 0.5, 1, 2, 3, 4, 5, 8, 12 and 24 h after injection. Concentrations of T4 were similar in serum from diestrous and pregnant bitches but were greater (P less than 0.025) than those in dogs of other reproductive states before and after treatment with TSH. Concentrations of T3 were greater (P less than 0.005) in serum from diestrous bitches before and after TSH injection than in serum from dogs of all other reproductive states. Concentrations of T3 in males, anestrus, proestrus, pregnancy and lactation did not differ. Basal concentrations of cortisol did not differ consistently among reproductive states. However, concentrations post-ACTH were different (P less than 0.05) with anestrus = diestrus greater than lactation = pregnancy = male greater than proestrus. These results indicate that reproductive state of experimental animals must be considered when studying thyroidal and adrenal functions.