1. Pupillary responses to locally applied phenylephrine and cardiac responses to intravenous hyoscine N-butylbromide have been studied in ten pairs each of healthy monovular and binovular twins aged between 24 and 60 years. 2. Under identical lighting conditions resting pupil diameters differed widely among individuals, being greater in female subjects and in those with larger irides and smaller with increasing age. 3. Monovular twins, but not binovular twins, showed significant concordance in iris diameter, initial pupil diameter, initial heart rate and responses to both drugs. The results suggest that these responses were dependent on the concordance of peripheral sympathetic and parasympathetic tone within each twin pair. 4. It is concluded that much of the variability in response to drugs acting on the autonomic nervous system is determined by physiological factors and that these are genetically influenced.