Captopril and aspirin have been claimed to adversely affect renal function. This study was designed to evaluate the safety of concomitant administration of both drugs in patients with moderate to severe congestive heart failure (CHF). The study group consisted of 10 patients with a mean age of 77.6 ± 4.4 years and a mean New York Heart Association functional class of 2.6 ± 0.5. Captopril was administered in a rapidly escalating dose regimen over a 4-day period to a maximum dose of 75 mg/day. Aspirin 0.25 g/day was added from day 5 on. Renal plasma flow (RPF) was measured by iodohippurate scan and the creatinine clearance test (Ccr) was used as an index of glomerular filtration rate (GFR). Both Ccr and RPF remained unchanged throughout the study period: 48.9 ± 16; 48.2 ± 16.5; 49.4 ± 16, and 222 ± 67, 241 ± 97, 237 ± 88 ml/min, for days 0, 4, 9, respectively. Only 1 patient developed a significant decrease in Ccr following the administration of captopril. This patient had a further decrease when aspirin was added. The decrease in Ccr was accompanied by a marked reduction in filtration fraction and in mean arterial pressure. Our data suggest that the administration of aspirin to elderly patients with moderate CHF treated with captopril is relatively safe and is not associated with further deterioration in renal function.