Ninety patients who had phacoemulsification and implantation of a flexible polyHEMA intraocular lens (IOGEL 1103) were assigned to three groups. After evacuation of sodium hyaluronate retrolentally from the capsular bag, 0.5 ml of 1% acetylcholine chloride, 0.01% carbachol, or balanced salt solution was instilled into the anterior chamber following wound closure. Intraocular pressure (IOP) was measured the day before, and six hours, 18 hours, and one week postoperatively. No topical or systemic antiglaucomatous drug was given during the study period. Preoperatively and one week postoperatively there was no significant difference between the three groups (P greater than .01). At six hours postoperatively the mean IOP decreased in the carbachol group (-2.8 mm Hg) and increased in the acetylcholine and control groups (+0.6 mm Hg and +/- 4.7 mm Hg) when compared with baseline pressures. At 18 hours the mean change from baseline was -3.0 mm Hg in the carbachol group, +0.8 mm Hg in the acetylcholine group, and +2.3 mm Hg in the control group. At six hours IOP exceeding 22 mm Hg was observed in ten of the control patients (30%) receiving balanced salt solution intracamerally and four of the acetylcholine patients (13.3%) but none of the carbachol patients. At 18 hours IOP remained above 22 mm Hg in three of the acetylcholine patients (10%) and four of the control patients (13.3%). Only one of the carbachol patients developed an increase of IOP up to 26 mm Hg at 18 hours. Removal of viscoelastic substances from behind the IOL reduced the incidence of pressure spikes in the early postoperative period.(ABSTRACT TRUNCATED AT 250 WORDS)