The effect of formulations of guanethidine and adrenaline of different composition has been tested in rabbits and in patients with glaucoma. The concentrations of guanethidine and adrenaline used for the rabbits were 5.0% and 1.0%; 2.5% and 0.5%; 1.0% and 0.2%; 0.5% and 0.1%. All except the lowest combination were equally effective in the magnitude of the decrease in intraocular pressure brought about and in their duration of activity. Two formulations containing guanethidine and adrenaline at concentrations of 3.0% and 0.5% and 1.0% and 0.2% respectively (formulated as Ganda drops by Smith and Nephew Pharmaceuticals Ltd) were tested in a blind, cross-over, short-term clinical trial on 20 patients. The drops containing the lower concentration of drugs were as effective as those of higher concentration. These results lead us to believe that most patients who respond to this treatment could be put on a reduced dosage regimen, which should result in a decreased incidence and severity of side effects.