The acute effects of topical pilocarpine 4 per cent, administered to one eye, have been studied in replicate experiments in young volunteers. One hour after medication administration the intraocular pressure was significantly lower. The magnitude of the reduction exceeded that predicated from the significant increase of true facility of outflow. This is explained by the observation of a significant reduction of calculated aqueous humor flow. Pilocarpine caused a significant increase of pseudofacility, a clinically useful drug effect not previously reported. One hour after medication administration, measured episcleral venous pressure was not different from the pretreatment level.