Ejaculates produced by 82 men between 6 and 19 days after removal of approximately 5 cm of vas deferens were assessed specifically for motility of residual spermatozoa. Several of 23 ejaculates produced 6 to 8 days after vasectomy still contained spermatozoa of the quality and number likely to produce pregnancy. By 13 to 15 days, however, all or the great majority of residual spermatozoa were dead, and in only 1 of 50 men were there numbers of motile spermatozoa possibly compatible with fertility. The results seem to justify more extensive surveys to confirm the absolute or biologic limits of the viability of spermatozoa in the terminal portion of the human male reproductive tract, anticipated here to be 16 to 18 days, as a suggested basis for a standard postvasectomy protocol. Second, these data support the choice of the 3rd postoperative week as the earliest rational moment to confirm the absence of motile spermatozoa, or in their presence to suspect an incomplete block.