During earlier fat cell studies we noticed that homogenates of white fat cells became more brown with training, a fact that might reflect an increased content of mitochondria. This raised the question whether training (as is the case in muscle) increases the oxidative capacity in fat cells. Groups of 8-12 rats were swim trained for 10 wk or served as either sedentary, sham swim-trained, or cold-stressed controls. White adipose tissue was removed, and the activities of the respiratory chain enzyme cytochrome-c oxidase (CCO) and of the enzyme malate dehydrogenase (MDH), which participates in the tricarboxylic acid cycle as well as in the mitochondrial malate-aspartate and acetyl-group shuttles, were determined. The CCO and MDH activities expressed per milligram protein were increased in male rats 4.4- and 2.8-fold, respectively, in the swim-trained compared with the sham swim-trained rats (P less than 0.05). In female rats the CCO activity expressed per milligram protein was increased 4.5-fold in the trained compared with the sedentary control rats (P less than 0.01). Neither cold stress nor sham swim training increased CCO or MDH activities in white adipose tissue (P greater than 0.05). In conclusion, in rats, intensive endurance training induces an increase in mitochondrial enzyme activities in white adipose tissue as is seen in skeletal muscle.