Carboplatin 360 mg/m2was administered intrapleurally to 4 patients with malignant pleural effusion. Simultaneous pleural fluid and plasma samples were collected at various times after treatment to measure the total and filterable levels of platinum. The pleural area under the concentration versus time curve (AUC) for filterable platinum was 26- to 98-fold greater than the corresponding plasma AUC, plasma levels remaining in the range of 3 to 18 μmol/L. The concentrations of platinum in plasma were compared with those observed in 7 additional patients receiving the same dose of carboplatin intravenously. The comparative intrapleural and intravenous therapy led to peak concentrations of platinum 6 to 7 times lower after intrapleural than after intravenous treatment, in significantly lower partial AUCs, and in prolonged levels of filterable platinum following intrapleural treatment. No significant difference, either for total or for filterable platinum, was noted comparing the overall plasma exposure. Systemic clearance and mean residence time values for filterable platinum in plasma were about 1.8 times lower and 2 times higher, respectively, after intrapleural treatment than after intravenous administration. Both the cumulative urinary excretion and renal clearance, as well as the apparent volume of distribution at steady-state for filterable platinum, were comparable with the values obtained after intravenous carboplatin. The observed differences in plasma pharmacokinetics between the 2 routes of administration mainly reflect the delayed absorption of carboplatin into the systemic circulation after intrapleural treatment. The locoregional treatment was well tolerated by all patients, and no symptoms of thoracic discomfort or acute toxicity were noted.