We studied the pupillary cardiovascular and gastrointestinal effects of two parasympathetic blocker mydriatics. Thirty-four neonates were randomly assigned into 3 groups: A: Atropine sulfate 0.3%, B: Tropicamide 0.5%, C: placebo. Mydriasis was obtained in groups A and B (p less than 0.001). No hypertension was observed and only an increase in heart rate with atropine was significant (p less than 0.001). Gastrointestinal side-effects studied in 25 children revealed a disturbance in groups A and B as compared to placebo (p less than 0.01). Tropicamide is therefore a more useful drug in low birth weight infants due to the absence of cardiovascular and hypertensive side-effects. Those infants with gastrointestinal disease should be treated with caution due to the side-effects which may be encountered.