The structure and permeability of the testicular microvasculature in the adult golden hamster during different phases of gonadal activity was examined. After 12 weeks of exposure to a short photoperiod (SD; 6L:18D), maximal testicular regression with over tenfold reduction in size was achieved as compared with active testes of animals maintained in long photoperiod (LD; 16L:8D). Testes weights and volumes in regressed testes were not significantly different from the values measured in animals undergoing early recrudescence (transfer from SD to LD for 1 or 2 weeks). The volume density of testicular blood vessels and their lumina did not differ significantly between fully gonadally active, fully regressed animals or those transferred from SD to LD for 2 weeks. However, in animals transferred for 1 week from SD to the stimulatory LD, the density of testicular blood vessels and vascular permeability to the endothelial tracer horseradish peroxidase were significantly increased, as compared to all other groups. An angiogenic process was observed by electron microscopy, which was initiated in the regressed gonad and which was prominent 1 week after transfer from SD to LD, but it was less conspicuous 2 weeks after transfer from SD to LD. The angiogenic process was characterized by activated developing blood vessels with a basal lamina and a lumen, which was formed by dilatation of an interendothelial space. There were two types of endothelial sprouts: the first with one layer of basal lamina, indicating true neovascularization, and the second with additional layers of basal lamina. In the latter, the presence of a superfluous basal lamina indicates that regeneration takes place along the path of old vessels. In fully regressed animals isolated basal-lamina-like structures were observed. Basal laminae are known to survive endothelial cell death, and these basal laminae later appear to serve as a scaffold for regeneration of new vessels. The rapid renewal of the testicular microvasculature under physiological stimuli suggests that the recrudescing testis of the golden hamster can be viewed as a physiological model of angiogenesis.