In rats weighing 97-171 g, there is a significant negative correlation between the [Na] and [K] per g of water in freshly dissected epididymal adipose tissue and total body weight. During in vitro incubation in Krebs-Ringer's solution at 37°C, epididymal adipose tissue loses K and probably gains water and Na. After incubation, insulin-treated adipose tissue contains slightly more K and less Na than the corresponding control tissue. Glucose alone also increases the [K] of incubated adipose tissue. In the presence of glucose, the effect of insulin on adipose tissue K is considerably enhanced and the water content of the insulin-treated adipose tissue is significantly lower.