Experiments on male Wistar rat thymocytes have demonstrated that cyclosporin A, an active substance of the immunodepressive agent sandimmune given in concentrations of 1 to 10 micrograms/ml, has no effects on cellular oxygen consumption rates measured by the polarographic technique by using a Clarke electrode. At the same time cyclosporin A suppressed thymocyte respiration stimulated by the oxidative phosphorylation release 2,4-DNP (40 micrograms/M) by 18 and 56% when given by concentrations of 2 and 10 micrograms/ml, respectively. The relationship between the adverse effects clinically observed and the findings was also examined.