Total outflow facility was determined by two-level constant-pressure perfusion in cynomolgus monkeys for 30 to 40 min before and after intracameral infusion of cytochalasins B and D (CB, CD). CB and CD dose-facility response relationships were defined. CB doses of 0.1, 0.2, and 0.5 micrograms were subthreshold; 1, 2, 5 and 10 micrograms were effective, with 5 micrograms probably supramaximal. CD at 0.02 microgram was probably subthreshold; 0.1, 0.5, 1, 2.5, and 5 micrograms doses were effective, with 0.1 microgram being near or above maximum. Although the maximum effects of both agents were quantitatively similar, approximately doubling-starting facility, CD Was about 25 times more potent. Therefore, CB- and CD-induced facility increases are presumably due largely to interference with formation, structure, and function of actin filaments in the endothelial cells of the trabecular meshwork and inner wall of Schlemm’s canal.