Abstract In vivo administration of cyclosporin A (CyA) was found to determine some variations in nucleotide content of rat lymphocytes. ATP levels were reduced by CyA treatment, and the effect was more evident in peripheral blood than in spleen lymphocytes. In contrast, cAMP values were increased upon pharmacologic treatment with the same major evidence at the blood lymphocyte level. Intralymphocytic phosphodiesterase enzyme activity became detectable during CyA administration, whereas the intracellular redox state (NAD +/NADH ratios) did not vary significantly. These results were amplified by increasing CyA concentration.