The exposure of male rats to continuous chronic hypobaric hypoxia (HH) and intermittent chronic hypobaric hypoxia induced evident changes in testicular morphology and spermatogenic cell metabolism. The mechanisms that underlie these changes under HH are not known. In this work, we have tested the hypothesis that in rats subjected to HH, the testis undergoes changes in vascularization leading to changes in temperature homeostasis. Male Wistar rats (247 +/- 16 g) were maintained in normobaric or hypobaric (428 torr, equivalent to 4600 m a.s.l) conditions. At days 0, 5, 15, and 30 postexposure, 12 rats were anesthetized with ketamine, and the intratesticular temperature was determined. These rats were subsequently sacrificed and the testicles were fixed in formaldehyde and processed for routine histological analysis. Our results showed that the height of the seminiferous epithelium decreased significantly at day 5 posthypoxia and thereafter, indicating a decreased spermatogenesis. Intratesticular temperature increased (1.5 degrees C) and remained high after 5 days of hypoxia exposure. Correlated with these changes, histometrical analysis of the number of blood vessels in the testicular interstitium was significantly increased by day 5 and afterwards. Morphological classification of interstitial blood vessels indicates a transition from capillaries to larger vessels as the hypoxia exposure progresses.