Background: Latanoprost is a prostaglandin (PG)F2! analogue widely recognized in the treatment of glaucoma. To investigate the action of this drug on the ocular circulation, we have studied its effects on isolated rabbit ciliary artery. The data obtained on this drug are compared with the data from PGE2 and PGF2!. Methods: Under the microscope, ciliary artery specimens were prepared from rabbit eyes and mounted in a myograph system. The effects of latanoprost, PGE2, and PGF2! on the isolated rabbit ciliary artery were investigated in vitro using isometric tension recording methods. Results: Exogenously applied PGF2!, but not latanoprost, evoked contraction in the rabbit ciliary artery. After precontraction with excess-[K]0 solution, latanoprost evoked relaxation dose-dependently. Latanoprost at a concentration of 100µM induced maximum relaxation, which was not blocked by 10 µM L-nitro-L-arginine methylester (L-NAME), 1 µM 8-37 calcitonin gene-related peptide (CGRP) or 10 µM indomethacin. Moreover, latanoprost induced relaxation even in preparations without endothelium. The maximum relaxation obtained with PGE2 was somewhat less than 50% of that with latanoprost. Conclusions: These results indicate that latanoprost and PGE2 relaxed rabbit ciliary artery to different degrees. The relaxation provoked by latanoprost was not dependent on endothelium and was not caused by intrinsic PG, CGRP or nitric oxide. The mechanism of this relaxation is not yet clear.