A trial of a mixture of guanethidine 5% and adrenaline 0.5% (Ganda 5.05) and of guanethidine 3% and adrenaline 0.5% (Ganda 3.05) was conducted on 90 eyes in 53 patients with open-angle glaucoma or ocular hypertension. The cases fell into 5 groups: untreated cases, cases on pilocarpine 1%, on pilocarpine 2%, on pilocarpine 2 to 4% and adrenaline 1%, and on separate guanethidine 5% and adrenaline 1%. Baseline pressures and average pressures on the previous treatment were established. Substitution with Ganda 3.05 or 5.05 was started, and the patients attended 2 weeks, 1 month, 3 months, and 6 months from the start of the trial. Applanation tonometry was carried out at the same time of day. The pupil was measured, ptosis and superficial punctate corneal staining were looked for and evaluated, and the patients were questioned for symptoms of side effects and acceptability. All the eyes that had previously been treated with pilocarpine 1% or 2% presented significantly lower intraocular pressures on Ganda 3.05. The patients on pilocarpine 4% and adrenaline 1% also had lower intraocular pressures on Ganda 5.05, but the significance was less, and the patients on separate guanethidine and adrenaline had a small but not statistically significant drop in pressure. Ptosis and discomfort were evaluated on a subjective scale. Patient acceptability was good. The trial was interrupted in 5 cases for various reasons. Tachyphylaxis and tolerance to the mixtures were not observed in this series.