1. 1. Naringin at all doses (2 × 10 −6, 5 × 10 −7, and 1 × 10 −7 M) significantly increased contractions induced by noradrenaline in rat vas deferens but the increments of maximal contraction were not concentration-dependent. 2. 2. In a medium containing 1 × 10 −6 M yohimbine (a selective blocker of α 2-adrenoceptor) and naringin, the curve constructed with noradrenaline decreased below the control curve. 3. 3. Naringenin (aglycone of naringin) (2 × 10 −6 and 1 × 10 −7 M) increased the contractile effect of noradrenaline and the maximal effect evoked was related to the maximal dose of naringenin. 4. 4. The α 2 antagonism produced by yohimbine in the naringenin-noradrenaline association were retained at two doses of naringenin tested and we noticed a similar behaviour when we used clonidine-noradrenaline.