The effects of an intermittent high intensity stimulus (running) or a chronic low intensity stimulus (cold acclimation) of oxidative metabolism on maximal oxygen uptake (VO2,max), lung O2 diffusing capacity (DLO2) and skeletal muscle as well as fat tissue mitochondrial content in growing guinea pigs are described. Young male guinea pigs were assigned to three experimental groups (n = 5): control (C), endurance trained (T; at 70% VO2max) or cold acclimated (CA; 5-7 degrees C) for six weeks. Animals were sacrificed at the end of the experimental period and tissue for morphometric analysis of the lung, muscle and interscapular fat was sampled. T and CA animals significantly increased weight specific VO2max by 23% and 29%, respectively. Despite a significant increase in absolute lung volume in T (+10%) and in weight specific lung volume in CA (+20%) neither absolute nor weight specific DLO2 was significantly affected by the experimental treatments. In trained animals the total volume of mitochondria remained unchanged in samples representative for the entire musculature but was significantly increased in M. vastus intermedius (+72%). Intramyocellular lipids increased significantly both in M. vastus intermedius (+244%) as well as in the whole body musculature (+164%). Cold acclimation increased the mitochondrial content of the interscapular fat pad by approximately 20-fold but had no effect on total mitochondrial volume in muscle. We conclude that the increase in oxygen demand resulting from exercise training or from cold acclimation could be accomodated by the existing lung diffusing capacity and did not induce a global change of oxidative capacity in skeletal muscle tissue in growing guinea pigs. Exercise training caused oxidative capacity to increase only in a locomotor muscle activated during running whereas cold acclimation greatly increased interscapular fat tissue oxidative capacity.