The growth of the R3230AC mammary carcinoma was studied in female Fisher 344 rats that were treated with diazepam. Diazepam at 250 micrograms/animal/day (2.5 mg/kg body weight) and 500 micrograms/day was administered for 4 weeks, starting 2 days before subcutaneous transplantation of uniform pieces of the tumour. Tumour size was significantly increased in the group receiving 250 micrograms but was below the levels observed in the saline treated controls in those given 500 micrograms diazepam. Plasma PGE2 and PGF2 alpha concentrations were comparable in control and diazepam treated animals.