The membrane-bound Na/K-ATPase system is an important energy regulating system for all primate cells and is suspected to be either primary or secondary affected in different but important metabolic disorders as essential hypertension, diabetes II (mature onset diabetes) or severe overweight. Rabbit smooth muscle cells grown in culture have been incubated with different concentrations of ouabain (10(-7)-10(-4) mol/l) and potassium (4 and 6 mmol/l). In controlled series, the incorporation of 3H-thymidine (and collagen secretion) during incubation with ouabain was found to be diminished by at maximum 73% (P less than 0.01) and this was found to be reversible by changing to ouabain-free medium or partly by the addition of extra potassium. The intracellular ATP level and lactate production was diminished together with the fall in 3H-thymidine incorporation. These effects were probably not due to a non-specific toxic effect of ouabain because no difference in leakage of prelabelled 3H-thymidine from cells compared to control series was seen. We suggest that an optimal function of the membrane-bound Na/K-ATPase system is of great importance not only for intracellular energy production but also for cell proliferation and protein synthesis.