Effects of prostaglandins (PGs) E1, E2 and F2 alpha on the electrically stimulated and non-stimulated smooth muscles of bovine intra-ocular tissues were investigated in vitro. PGs E1, E2 and F2 alpha contracted the sphincter and these contractions were not antagonized by either cholinergic or adrenergic blocking agents. PGF2 alpha was much weaker than PGE1 or PGE2. PGs in high concentrations produced an irreversible contracture of the sphincter muscle. In contrast, the dilator muscle contracted with PGF2 alpha, but not PGE1 or PGE2. The ciliary muscle did not respond to these PGs or to indomethacin. Electrical field stimulation with short pulses produced excitation of the intrinsic nerves. The responses produced were not altered in amplitude by the application of any dose of PGs or indomethacin, although indomethacin did lower the tone of the sphincter. PGs did not influence the amplitude of the acetylcholine-induced responses of the intra-ocular muscles. These results indicate that in each of the bovine intra-ocular muscles, PGs play a minor role in neuromuscular transmission both prejunctionally and postjunctionally. Thus, PG-induced contractions of the bovine intra-ocular muscles are considered to occur by a direct action on the muscle rather than on the nerves.