A recent study of a potent new antihypertensive agent pheniprazine hydrochloride (Catron *) indicated that some of the patients treated with this drug developed color blindness and decreased visual acuity which improved spontaneously when the drug was discontinued.1This report deals in more detail with the ophthalmological findings in these and other patients and, in addition, pathological findings are presented which document the toxic nature of the visual defects described.Materials and MethodsPatients receiving pheniprazine hydrochloride who developed disturbances of visual function were seen in consultation by members of the staff of the Ophthalmology Branch at the National Institutes of Health. Each patient received a complete ophthalmological examination which included the evaluation of color vision with the Hardy-Rand-Rittler Pseudoisochromatic Plates and in some cases the Farnsworth D-15 Panel was used.Autopsy material available from one patient (Case 8) was fixed in 10% formalin 16 hours after death and studied