The pharmacokinetics of cisplatin were investigated in 14 patients, aged 10 months through 13 years who were affected by solid malignant tumors. High-dose cisplatin (40 mg/m2/d) was administered with repeated courses for five days as a continuous intravenous (IV) infusion. Total platinum (Pt) levels in plasma and urine and free (protein-unbound) Pt levels in plasma ultrafiltrate were determined by inductively coupled plasma atomic emission spectrometry (ICP-AES). Areas under the concentration v time curve (AUCs) for mean total and free Pt levels were calculated for the 120-hour period of infusion and for the 384 and 120 hours following its completion, respectively. Half-lives of total and free Pt in plasma were calculated for the 216 hours following completion of infusion in five patients at their first course. The fraction of the administered Pt dose excreted in urine as Pt was determined for the five-day period of infusion and seven-day period after its completion. A total of 36 courses were studied. Maximum average Pt levels were reached after 120 hours of infusion: at the first course, 3.22 and 0.17 micrograms/mL for total and free Pt, respectively. Platinum levels declined according to a biexponential model, with initial half-lives of 18.3 and 16.9 minutes, and terminal half-lives of 81.9 and 59.0 hours as determined for total and free Pt, respectively. In the second and third courses studied there was a progressive increase in mean Pt plasma levels. Consequently, the free drug exposure as measured by AUC increased in all patients with repeated courses: 47.7% for the second and 124.4% for the third course, when compared with the first. At the same time, the mean fraction of the dose excreted in the urine for the 12-day period considered, was 44.1% for the first course, 36.2% for the second, and 28.4% for the third. The progressive enhancement of tissue exposure to the free cytotoxic drug, resulting from a reduced renal clearance of Pt with sequential courses of cisplatin, produced mainly increased toxicity while therapeutic effect progressively diminished.