The effects of nitric oxide (NO) and noradrenergic activation in the posterior hypothalamus on arterial pressure tolerance induced by subcutaneous injection of nitroglycerin (NTG) was investigated in anesthetized Sprague-Dawley rats. Intravenous injections of NTG (3, 10, and 30 μg/kg) and sodium nitroprusside (1,3, and 10 μg/kg) produced dose-dependant decreases in arterial blood pressure. Tolerance to NTG was produced by subcutaneous administration of 4.0 mg of NTG as 4 separate hourly injections of 1.0 mg each, affecting the dose-dependent response of NTG IV injection. The 4 high-dose NTG pulse injections produced a marked shift in the dose—response curves for arterial pressure depression induced by intravenous injection of the challenge doses of NTG, but did not alter hypotensive responses to sodium nitroprusside. The tolerance responses to arterial pressure depression were enhanced by a bilateral microinjection of NTG (1 nmol) and by diethylamine NONOate (1 nmol), an NO donor, into the posterior hypothalamus. Bilateral microinjection of guanethidine (1.5 nmol), a noradrenergic blocker, into the posterior hypothalamus inhibits NTG tolerance in a period of time within 2 hours. We conclude that exogenous NO and noradrenergic activation in the posterior hypothalamus play an important role in arterial pressure tolerance to systemically administered NTG.