A method of measuring pupillary diameter in the dog using a Kowa RC-2 fundus camera, to which an eyepiece graticule had been fitted, is described. Pupil diameter was measured either directly by reading off from the calibrated graticule or from photographs. The pupillary diameters of 105 (53 male, 52 female) untreated English beagles aged between 8 and 32 months were measured under light intensities of 150-200 lux and 1000-1400 lux. Mydriatic and miotic pupillary diameters were measured in 62 male and 80 female dogs. Time-effect curves were plotted for two groups of 5 beagles receiving 75 mug/kg or 175 mug/kg atropine and 100 mug/kg or 300 mug/kg propantheline intravenously. The dose-effect curve for propantheline was found to be linear. A dose of 109 (97-121) mug/kg propantheline increased pupillary diameter by 3 mm, 20 min after administration. The time- and dose-effect curves, the significant sex differences and age dependency seen in this study indicate both a high degree of accuracy and the suitability of both of these methods for use in pharmacological and toxicological studies.