Vasoactive-intestinal-peptide (VΙP)-containing nerve fibers impinge upon both follicle cells and blood vessels in the thyroid gland. We have previously shown that VIP induces a specific, dose-related increase in thyroid blood flow in the rat. However, our VIP treatments had no effect on circulating thyroid hormone levels. Since a number of reports have indicated that VIP can enhance thyroid hormone secretion, we have expanded our studies to characterize more completely the conditions under which VIP might stimulate thyroid hormone secretion in the rat. In unanesthetized, unstressed rats with chronic catheters, 33 µg VIP/100 g body weight failed to alter triiodothyronine (TO or thyroxine (T4) levels and did not affect the thyroid secretory response to a submaximal dose of bovine TSH. In euthyroid and hyperthyroid rats, the release of 125I was increased after exogenous TSH, but was not altered by VIP. The only condition in which we observed a rise in circulating T3 levels in response to VIP was during a continuous 2 h infusion of a high dose (0.25 µg/min, i.v.) of this peptide. However, plasma TSH levels tended to be elevated in these rats, suggesting an indirect effect via TSH. This suggestion is strengthened by our observation that VIP failed to alter T3 or T4 release after topical application (0.1 µg/µl for 3 h) in vivo or after in vitro treatment (10–6M for 4 h), even though these preparations were fully responsive to bovine TSH. Despite the lack of a thyroid hormone secretory response, VIP did stimulate cyclic AMP release in vitro, indicating that at least some component of the thyroid is responsive to this peptide. Thus, while it appears that VIP can elicit thyroid hormone secretion, such effects are observed only under treatment conditions which greatly exceed those shown to modulate thyroid blood flow. This leads us to believe that thyroidal VIP has little or no importance in the normal physiological regulation of thyroid hormone sec