An ocular hypertensive rabbit eye model was used to study the effect of L- and D-timolol on retinal and choroidal blood flow. Ocular hypertension was induced artificially by raising the intraocular pressure to 40 mmHg which reduced the ocular blood flow to 1/3 that of the normal blood flow. The effects of L- and D-isomers of timolol on the ocular blood flow of ocular hypertensive eyes were then determined using the colored microspheres technique over a range of 0-180 min after eyedrop instillation. It was found that L-timolol produced biphasic action on the blood flow in the iris, ciliary body and choroid by significantly reducing it initially at 30 min, then markedly increasing it later at 90 min and thereafter. Although L-timolol produced a tendency to reduce the blood flow in the retina, it was not statistically significant. At 90 min and thereafter, the blood flow in all ocular tissues was markedly increased by L-timolol. D-timolol did not reduce the blood flow of ocular tissues initially. There was a tendency to increase the blood flow in all ocular tissues from the very beginning by D-timolol. The increase of blood flow became significant at 90 min and thereafter, up to 3 hrs. D-timolol was less potent than L-timolol in increasing the blood flow in ocular tissues.