Over a period of 4 months, 16 of 24 patients (30 of 46 eyes) with either primary open angle glaucoma (POAG) or suspected glaucoma were treated successfully with a maintenance dose of guanethidine (3%) and adrenaline (0.5%) combined in one eyedrop (GA) once daily. In the previous month the medication was given twice daily and at the end of 4 months the decrease in intraocular pressure (IOP) was 8.9 mm Hg (33%) compared to 9.9 mm Hg (37%) with twice-daily application; three of four eyes responded just as well to single-daily application as to twice daily application of GA. Once-daily treatment with GA was not successful when the average IOP in the absence of treatment was over 32 mm Hg. The advantages of once-daily application were less conjunctival hyperemia, less dilation of the pupils, less ptosis and difficulty reading, plus the advantage that the drops needed only to be applied once a day (patient compliance). The recommended regime for GA therapy in patients with an IOP of less than 32 mm Hg is application of GA twice daily for 1 month followed by a decrease in the dosage to once a day. GA can best be applied in the evening before retiring.