The intraperitoneal (i.p.) administration of cisplatin (CDDP) is one of the most effective therapies For cancers that are confined to the abdominal cavity. However, the effect of fluid osmolarity on the therapeutic efficacy of i.p. administration of CDDP has not been well established. In the current study, hypotonic (154 mosmol l(-1)), isotonic (308 mosmol l(-1)) and hypertonic (616 mosmol l(-1)) solutions of CDDP were prepared for an evaluation of their therapeutic efficacy in an experimental system. After i.p. administration. uptake of CDDP in vivo by tumour cells in hypotonic solution was significantly greater than in isotonic or hypertonic solution. The 50% lethal dose (LD(50)) value of CDDP in hypotonic solution (12.1 mg kg(-1)) was lower than that in isotonic solution (16.9 mg kg(-1)) and than that in hypertonic solution (28.6 mg kg(-1)). However, when a dose equal to one-half of the LD(50) was administered in each solution to mice with i.p. tumours, survival of mice given the CDDP in hypotonic solution was significantly prolonged as compared with the survival of the Other mice. These results demonstrate that the therapeutic efficacy of i.p. CDDP in mice is augmented when the drug is administered in hypotonic solution.