To determine if acute or chronic (21 days) losartan (10 mg/kg, s.c.) regulates the renin-angiotensin system in interscapular brown adipose tissue, angiotensin II (AII) content and [3H]overflow from slices preloaded with [3H]norepinephrine were examined. Acute or chronic losartan administration had no effect on AII content. AII increased evoked [3H] overflow from slices from control rats. Losartan administration did not alter basal [3H]outflow or evoked [3H]overflow. Acute losartan administration inhibited AII-induced enhancement of evoked [3H]overflow. Tolerance developed to the inhibitory effect of losartan following chronic administration.