The effect of alpha-human atrial natriuretic peptide (alpha-hANP) on the intraocular pressure (IOP) was studied in normal rabbits. Dose response studies to IOP were performed after intravenous administration of alpha-hANP at 6.25, 12.5, 25.0, 50 and 100 μg/kg to the rabbits. The maximum reduction in IOP was observed to occur with the alpha-hANP 50 μg/kg dose. The time course of the pressure response to alpha-hANP were measured prior to and following intravenous administration of 50 μg/kg of alpha-hANP. The change in IOP and blood pressure showed the same pattern in the simultaneous record. Silver grains of 125I-alpha-hANP (10 μCi/g body weight) were detected in the ciliary processes of mice by radioautography after intravenous injection. The pupillary response was not tested in this experiment. The mechanism of IOP reduction was discussed.