The effects of atriopeptin (AP) on intraocular pressure (IOP), aqueous humor formation (AHF) and ciliary cyclic GMP in bovine eyes perfused in vitro through a ciliary artery were investigated. AHF was monitored fluorophotometrically by perfusing the anterior chamber with Barany's mock aqueous humor containing fluorescein. To study the effect of AP on cyclic GMP synthesis, eyes were injected with an intra-arterial bolus of AP prior to dissection of the ciliary processes. For comparison, individual ciliary processes, or isolated ciliary epithelial cells were incubated with AP. Cyclic GMP was extracted by homogenization, aqueous partition and column chromatography, and measured by radioimmunoassay (RIA). Decreases in IOP or A HF were seen (~)15 min after the injection of AP and persisted throughout a 60-80 min perfusion. Arterial perfusion pressure was not significantly altered by AP, even when vascular tone was raised by adding noradrenaline to the perfusate. In whole eyes, ciliary cyclic GMP increased in response to AP. Cyclic GMP in isolated ciliary processes, or cultured ciliary epithelial cells incubated with AP also increased. We conclude that the decrease in IOP in the bovine eye by AP is due to a reduction in AHF, which is independent of any vascular effect. Ciliary cyclic GMP may be the intracellular mediator involved.