Percutaneous absorption of testosterone was determined in newborn rhesus monkeys, an animal model which is relevant to man. Mean percentage of absorptions of 4 and 40 μg/cm2in the newborn were, respectively, 22.5 ± 2.2 (SD) and 6.8 ± 2.1. Statistical comparisons (Student'st-test) of these results with those obtained with adults show no significant difference (P> 0.05) in skin penetration of testosterone in newborn and adult rhesus monkeys. In the newborn, the efficiency of absorption (percentage) decreased when the topical dose was increased 10-fold. However, the total compound absorbed per cm2 area of skin actually increased from 0.9 to 2.7 μg.With one other newborn rhesus, a topical dose of 40 μg/cm2was applied to the ventral forearm and the area was occluded for 24 hr. Percutaneous absorption was 14.7%, a value twice that from nonoccluded absorption.Systemic absorption from a topical dose becomes critical in the newborn because the ratio of surface area (cm2) to body weight (kilograms) in the newborn is 3 times that in the adult. Given equal application area of skin per newborn and adult, the systemic absorption in the newborn becomes 3 times that of the adult when based on kilograms body weight. With a different ratio of skin surface to body weight, the therapeutic ratio probably is lower in the newborn than in the adult when the compound is applied topically.SpeculationA high percentage of a steroid compound can be absorbed through the skin of a newborn as well as an adult. The systemic availability of a drug to the newborn following topical application can be 3 times that of the adult, per unit of body weight.