Pupils are often dilated for examination the day before surgery and again on the day of surgery. The following experiment was performed to determine the effect of serial doses of two commonly used mydriatic agents: on two consecutive days the pupil of one of the eyes of 28 subjects was dilated with tropicamide 1%, and the pupil of one of the eyes of 30 subjects was dilated with cyclopentolate hydrochloride 1%. The other eyes in both groups were dilated only on the second day, and thus served as controls. Pupil sizes were measured from photographs before and after dilation. The pupils of the eyes treated twice with either drug did not dilate as well after the second dose as those of the control eyes (P less than .005 for tropicamide, P less than .001 for cyclopentolate). The pupils of the eyes twice-treated with tropicamide were an average of 0.15 mm smaller than the control pupils; those twice-treated with cyclopentolate were 0.36 mm smaller. For subjects treated with cyclopentolate, this decreased mydriasis was related to age (P less than .05) and to eye color (P less than .025): the younger and blue-eyed subjects dilated less on the second day than the older and brown-eyed subjects. If full mydriasis is required at surgery, pupils should probably not be dilated with either tropicamide or cyclopentolate the day before surgery.