Accommodative and pupillary responses, refractive change, and visual acuity after 30 microliters instillation of cyclopentolate hydrochloride of various concentrations (0.00625-0.1%) in 6 young normal males (27.0 +/- 0.6 yrs) were investigated. Both accommodative and pupillary responses were measured by an infrared optometer, at pre-instillation and 1, 3, 6, 9 and 24 hours post-instillation. No significant effects were induced by concentrations of cyclopentolate hydrochloride 0.0125% or less. Reduced amplitude of accommodation and enlarged pupil were, however, observed by 0.025% and 0.05% of cyclopentolate hydrochloride from 1 through 9 hours post-instillation. Miotic response in accommodation was also reduced from 1-6 hours at the same concentrations. A concentration of 0.1% cyclopentolate hydrochloride showed both reduction of accommodation amplitude and enlarged pupil for 24 hours after instillation. However, neither refraction nor visual acuity was affected by any concentration of cyclopentolate hydrochloride. The results suggest that cyclopentolate hydrochloride at concentrations up to 0.05% applied before sleep would not be accompanied by inhibitory effects on accommodation during the day.