The mydriatic and cycloplegic effects of either one drop of 0.1% cyclopentolate or two drops of 10% phenylephrine were studied over a period of up to 6 hours after drug instillation, using 5 subjects aged between 25 and 43 years. The extent of mydriasis was determined from the horizontal diameter of the pupil. Dynamic accommodation responses were monitored with an infra-red, continuously recording optometer. Static measurements of the accommodation response/stimulus curve were made with a laser optometer. With either drug, dilation of the pupil occurred more rapidly than recovery of normal pupil diameter. Both drugs caused alterations in the dynamic and static accommodation responses. Response times were generally slowed, and the slope of the accommodation response/stimulus curve and the amplitude of accommodation were reduced. The time course of these changes was broadly similar to that of the pupil dilation. Cyclopentolate hydrochloride was more efficient as a mydriatic but exercised a greater unwanted cycloplegic effect.