The cytotoxic and cytokinetic effects of prostaglandin (PG)D2 on human and rat glioma cells were studied in vitro, and the morphological changes occurring in glioma cells following PGD2 treatment were investigated by light and electron microscopy. The cytotoxic effect was evaluated by both colony formation assay and cell growth inhibition. The cytokinetic effect was analyzed by DNA histogram using a flow cytometer. It was found that PGD2 had a dose-dependent cytotoxic effect on the glioma cells and that rat C6 cells were less sensitive to PGD2 than human KY cells. Within 24h after treatment of KY cells with PGD2, a time-dependent cytotoxic effect was also observed. However, the effect of PGD2 on glioma cells was reversible at lower concentration. Investigation of KY cells treated with 2.5 micrograms/ml of PGD2 revealed that the cells in S and G2 + M phases gradually decreased in number and subsequently almost all cells were accumulated in the G0 + G1 phase 12 h later. However, no chronological change of the DNA histogram was obvious in the cells treated with a relatively high concentration of PGD2 (10 micrograms/ml). On the other hand, the treated cells showed more severe morphological changes at a higher concentration of PGD2. Glioma cells treated with PGD2 became round with cytoplasmic vacuolization and blebbing. Thus, PGD2 appears to be useful as an adjuvant chemotherapeutic agent for malignant glioma.