• We studied the effect of reducing eye drop size on the efficacy and systemic absorption of topical 2.5% phenylephrine hydrochloride in neonates and infants. Eleven neonates received an 8-μL drop volume in one eye and a 30-μL drop volume (commercial size) in the fellow eye. Mean pupillary dilation at 60 minutes was equivalent (4.86 mm vs 4.57 mm) for both eyes, respectively. The plasma phenylephrine level was determined for the two drop sizes in a second group of infants. Eight infants received an 8-μL drop volume in both eyes, while nine infants received a 30-μL drop volume in both eyes. the mean phenylephrine level at ten minutes was 0.9 ng/mL for the 8-μL drop group and 1.9 ng /mL for the 30 μL drop group. In neonates and infants, reducing the drop volume of topical phenylephrine may improve the risk-benefit ratio.