The pupils of neonates often need to be dilated to examine the retina for retinopathy of prematurity and other disorders. It is known that low-weight infants (less than 1600 grams) are susceptible to systemic hypertension when 10% or 2.5% phenylephrine eye drops are used. To find the safest and best commercially available mydriatic agent in neonates, 30 low-weight infants were divided evenly into three groups. The drops tested were cyclopentolate 0.5% alone, cyclopentolate 0.5% plus mydriacyl 0.5%, and a combination drop of phenylephrine 1% and cyclopentolate 0.2%. There was no clinically significant effect of any of the drops on systolic blood pressure or pulse rate. The cyclopentolate and phenylephrine combination dilated the pupils by a mean of 2.8 mm which was statistically greater than the other groups (P less than 0.01) and had a longer duration of maximal dilation than the other drops (P less than 0.05).