This study investigated the status and sensitivity of mitochondrial permeability transition (mPT) pore in testis and liver of rats exposed to doses of galactose, an acceptable model used to mimic natural aging. Male albino rats were divided into five groups of eight animals in each group for in vivo studies and administered distilled water, 50,100, 200 and 500 mg galactose/kgbdwt, respectively, for six consecutive weeks. Mitochondria were isolated from liver and testis by differential centrifugation. Mitochondrial permeability transition (mPT) was assessed as mitochondrial swelling and was monitored spectrophotometrically. Mitochondrial lipid peroxidation, ATPase activity, antioxidant enzymes, caspase activation, interleukins, and sperm functional characteristics were also assessed. Administration of galactose (50-500 mg/kg) to male Wistar albino rats had no effect whatsoever on the testicular mPT pore. However, liver mPT pore was significantly opened. Furthermore, the enhancement of mitochondrial ATPase activity and malondialdehyde generation were observed in the liver of galactose-exposed rats. Significant alterations in antioxidant enzymes were observed in post-mitochondrial fraction (PMF) of liver and testis. There were also increases in serum levels of IL-1β and 6. In addition, caspases 9 and 3 were significantly elevated in PMF of the liver with evidence of DNA fragmentation. However, there was no significant difference in levels of caspases in PMF of testis in model groups of galactose when compared with control. These results provide evidence that testis mitochondria do not readily undergo permeability transition pore upon exposure to doses of D-galactose that induce the opening of the pore in the liver.HighlightsTesticular mitochondria are less sensitive to induction of permeability transition than liver mitochondria in rats exposed to D-galactose for 6 weeks, despite the occurrence of alterations in the antioxidant defense system and generation of ROS in sperm cells as in hepatocytes.The occurrence of mitochondrial permeability transition in liver of galactose-exposed rats is consistent with malondialdehyde production, alteration in antioxidant levels, enhanced ATPase activity, caspases-9 and 3 activation, immune dysfunction, and DNA fragmentation.The study of biochemical basis of reduced sensitivity of testis to permeability transition under conditions which the liver is extremely susceptible may become useful in age associated-neurodegenerative diseases where apoptosis is upregulated and has to be properly managed to achieve downregulation.