The dynamics of aqueous humor were studied before and after discontinuing timolol maleate in chronic users. Nineteen patients (mean length of treatment, 44 months) were studied by measuring their baseline intraocular pressure (IOP) and aqueous humor flow during treatment and 2 days to 6 weeks after discontinuing the drug. Flow did not increase significantly until 4 days after discontinuing timolol, and IOP did not increase significantly until 14 days later. Flow did not revert to its normal rate for 2 to 6 weeks. Intraocular pressure was 15% lower and the flow 24% lower during maintenance treatment than 1 month after discontinuing timolol. This comparison demonstrates the efficacy of chronic treatment. Recovery of aqueous flow is slow after stopping timolol. This finding suggests that lower or less frequent doses than are customarily used might be equally efficacious. If timolol is discontinued before filtration surgery, its effects will remain for 2 to 4 weeks.