Abstract Recent studies from this laboratory showed that l-m-synephrine (phenylephrine), a metabolite of l-m-octapamine, inhibited the drinking response of rats to peripherally administered angiotensin II. The objective of this investigation was to determine whether the isomers of both octapamine and synephrine could inhibit angiotensin II-induced dipsogenesis in the rat. Of the isomers tested, only d, l-m-octopamine and l-m-synephrine blocked the dipsogenic response to administration of angiotensin II (200 μg/kg, SC). The antidipsogenic effect of both d, l-m-octopamine and l-m-synephrine could be blocked by concurrent administration of yohimbine (300 μg/kg, IP), an α 2-adrenoceptor antagonist. The results indicate that m-octopamine and m-synephrine exert their antidipsogenic effect via α 2-adrenoceptors. These studies add to a growing body of data suggesting that activation of α 2-adrenoceptors inhibits, while blockade of these receptors enhances, angiotensin II-induced drinking.