The mechanism for renal handling of carboplatin was studied in 17 ovarian cancer patients treated with a combination of carboplatin and cyclophosphamide. Carboplatin and [51Cr]-ethylenediaminetetraacetic acid (EDTA) renal clearances were measured simultaneously during short intervals of from 45 to 120 min. A total of 131 clearance intervals were analyzed during 35 chemotherapy courses. The carboplatin/[51Cr]-EDTA clearance ratio (R) served as an indicator of the net tubular reabsorption (R less than 1) or secretion (R greater than 1). The R value was calculated for each sampling interval. No significant difference was found between interpatient and intertreatment variation. The intertreatment variation as tested against the variation in the short intervals by an F-test was highly significant. We calculated the average R value for each treatment and consequently based our results on a total of 35 observations. The mean R value was 0.77 (t-test for R = 1; P less than 0.001). We conclude that the renal elimination of carboplatin takes place by glomerular filtration followed by tubular reabsorption.