In this paper, we report experiments examining the effect of cyclosporine A on "run-off" translation in microsomes isolated from tissues of Sprague-Dawley rats. In microsomes isolated from rat brain, kidney and thymus, cyclosporine A added in vitro in concentrations of up to 100 micrograms/ml did not reduce [3H]L-leucine incorporation relative to controls. A small dose-dependent reduction in [3H]leucine incorporation was observed in microsomes isolated from rat liver when cyclosporine A was added in high concentrations (5 and 6% at 25 and 100 micrograms/ml). However, when cyclosporine A was injected at 50 mg/kg/day for 10 days, [3H]L-leucine incorporation was inhibited 99.9% in microsomes isolated from kidney. The oral administration of cyclosporine A at 50 mg/kg/day for 6-10 days produced a 75% inhibition of incorporation by isolated renal microsomes. These changes were observed in the absence of measurable reductions in "run-off" transcription measured as [3H]UTP incorporation by renal nuclei exposed to cyclosporine A in concentrations of up to 100 micrograms/ml in vitro or isolated from animals given oral cyclosporine A at 50 mg/kg/day for 6 days. Cross-over experiments were performed using microsomes and microsomal supernatant fractions (cell saps) from tissues of animals treated with cyclosporine A and control vehicle. Renal cell sap from cyclosporine A treated animals inhibited [3H]L-leucine incorporation by microsomes isolated from the kidneys or other tissues of animals treated with control vehicle. These experiments demonstrated that a translation inhibitor was present in the cell sap of cyclosporine A treated animals which could directly block translation elongation in microsomes from control animals. When renal cell sap from both control and cyclosporine A treated animals was added to control microsomes, inhibition was still prominent, suggesting the presence of an inhibitor rather than the absence of an elongation factor. Oral administration of cyclosporine A at 50 mg/kg/day for 6 days depressed renal microsomal [3H]L-leucine incorporation equally in male and female rats to 25% of control. The dose-response relationship for microsomal protein synthesis inhibition after 6 days of oral cyclosporine A administration was: 5 mg/kg, 73.7% of control; 10 mg/kg, 64.1% of control; 25 mg/kg, 54.9% of control and 50 mg/kg, 24.1% of control. Renal microsomal protein synthesis following oral cyclosporine A at 50 mg/kg/day was reduced to 54% of control by day 2 and was maximally inhibited at 25-30% of control by day 4.(ABSTRACT TRUNCATED AT 400 WORDS)