In this prospective study we used the oculopressions-tonometry (OPT) for determination of the aqueous humor outflow facility (C). 53 phacic eyes with glaucoma simplex were examined pre- and postoperatively. One hour after operation C decreased from 0.15 to 0.14. In correlation to this fact the IOD increased. After 7 days C was higher already than preoperatively, 30 days and 6 months after operation C showed normal values. After 6 months the mean decrease in IOD was 5.8 mm Hg, the mean increase in C was 39%. The rate of success after 30 days was 77.4%. IOP and C showed statistically significant changes. The problems with determining C and the causes of the fluctuation of C during our follow-up are discussed.