SUMMARY By means of unilateral ligation of the epididymis and experimental cryptorchidism in twenty-one adult male rabbits, the reaction of spermatozoa in proximal regions of the epididymis to the effects of retention in the abdomen was investigated. Combined estimates of eosinophilic spermatozoa and decapitated spermatozoa in smears stained with aqueous nigrosin eosin were used to indicate the response in different samples. Examination of spermatozoa withdrawn from four levels of the tubule showed that those in the caput epididymidis and the initial segment reacted rapidly to the effects of artificial cryptorchidism, whilst the response of spermatozoa from the corpus epididymidis appeared to be delayed. Contralateral testes and ligated epididymides were maintained in the scrotum to serve as controls, and under these circumstances spermatogenesis continued for at least 3 weeks although, shortly after ligation, the epididymides were tightly packed with spermatozoa and fluid. Examination of the tubular contents in these specimens by means of the nigrosin eosin technique revealed a progressive degeneration of spermatozoa above the site of ligation. However, microscopical observations of motility indicated enhanced activity in samples collected during the first 3 weeks of ligation of the epididymis.