When PGD2 was topically applied to the rabbit eye a reduction of intraocular pressure (IOP) was observed within 30 min. The IOP reduction lasted throughout the observation period of 7 hours. No increase of IOP was observed during the period. The effect of PGD2 was dose-dependent (0.4 microgram to 250 micrograms), and the minimum effective dose was estimated to be 2.0 micrograms. The application of PGD2 did not affect the pupil diameter. No sign of inflammatory response was observed by PGD2 application, that is, no hyperaemia, no flare, no irritation, and no increase of protein content in the aqueous humour. These results suggest that PGD2 or its analogues may be useful for treating glaucoma.