Semen parameters were analyzed in a series of 76 men pre- and postvasectomy. The mean volume decrease was 0.66 ml but it was 0.5 ml or more for only 53% of subjects. A decrease in the number of spermatozoa was correlated primarily with the number of ejaculations. In nearly all cases the total number of spermatozoa contained in all semen samples examined after vasectomy was clearly lower than the average number contained in only one ejaculation prior to vasectomy. Motile spermatozoa were never observed after the 15th day following vasectomy. The reappearance of motile spermatozoa after that time was an almost certain sign of a defect in the vas block or of recanalization of the vas deferens.