Micro oxygen electrodes were used to measure PO2 gradients in rat testes and reproductive tracts. PO2 (mean mm Hg ± SD) in 14 rats was 15.2 ± 3.5 in interstitial tissue, 12.1 ± 3.3 in tubular lumina, 18.1 ± 5.4 in rete testes, 23.2 ± 2.5 and 24.3 ± 1.3, respectively, in caput and cauda epididymides and 23.1 ± 3.6 in vasa deferentia. Mean testicular vein (TV) PO2 in six similarly anesthetized rats was 31.3 ± 10.6 mm Hg. In 12 conscious rats with a mean testicular blood flow of 25.5± 7.0 ml/100 g/min, blood PO2 was 86.6 ± 4.7 and 53.4 ± 8.5 mm Hg for femoral artery (FA) and TV blood, respectively, yielding estimates for oxygen uptake of 116-148 µ[unknown] O2/100 mg of testis tissue/h. In five rats (testicular blood flow 19.8 ± 4.8 ml/100 g/min) the blood CO2 content was 38.6 ± 5.9 and 48.3 ± 2.9 ml/100 ml blood for FA and TV, respectively, yielding a CO2 production rate of 114 ± 57 µ[unknown]/100 mg testis tissue/h.