We investigated the pharmacokinetics of cyclosporin A (CsA) blood levels and drug toxicity in a chronic rat study in which long‐term (30 weeks) CsA was administered. Ninety Lewis rats received subcutaneous CsA at 5 mg/kg/day for 12 weeks, at which time CsA injections were stopped in 50 animals. The remaining 40 rats were maintained on 5 mg/kg CsA daily until week 18 and then switched to an alternate day dosing until week 30. All rats were observed daily and weighed weekly. Whole blood CsA levels were determined by a commercially available radioimmunoassay kit. The daily dosing regimen resulted in greatly elevated trough CsA levels (>1,600 μg/liter) and substantial chronic systemic toxicity, with weight loss and death in eight animals. Alternate day dosing reduced trough levels (mean 1,311 μg/liter) and decreased toxicity. Chronic administration by the subcutaneous route resulted in a considerable depot effect, with constancy of drug levels over a 48 hr dosing interval and a slow decline of drug levels (15 days) upon cessation of treatment. These results underscore the importance of monitoring both body weight and blood CsA levels in rodent studies when CsA is employed. Investigators should be aware of drug accumulation with chronic therapy and the consequent need to modify dosing to prevent toxicity. © Wiley‐Liss, Inc.