The prostatic half and epididymal half of the rat vas deferens was found to show the difference with respect to both the sensitivity (expressed as the geometric mean ED50) and maximal response to norepinephrine and acetylcholine. The epididymal half was more sensitive to both drugs than the prostatic half; the maximal response to norepinephrine was greater in the epididymal half than in the prostatic half. Effects of denervation, 10-6 M cocaine and chronic administration of reserpine (1 mg/kg/day for 5 days) on the responses to the drugs were also found to be qualitatively and/or quantitatively different between the two halves. In the epididymal half, all these procedures produced supersensitivity to acetylcholine as well as to norepinephrine. Whereas, in the prostatic half, supersensitivity to only norepinephrine was observed; the responses to acetylcholine were more or less depressed, accompanied by little change in the sensitivity, by these procedures. Degree of supersensitivity (ED50 control/ED50 treated) to norepinephrine produced by denervation was greater in the prostatic half than in the epididymal half. However, in the case of cocaine or reserpine, there was little difference in the degrees of the two halves. In both halves, the maximal response to norepinephrine was increased by denervation and cocaine, but not by reserpine. The magnitude of such increase in the prostatic half was greater and less, after denervation and cocaine, than that in the epididymal half, respectively. Only cocaine increased the maximal response of the epididymal half to acetylcholine. These results are discussed in relation to those previously obtained using whole vas deferens.