The permeability of the blood-brain barrier to several classes of compounds was studied in rats between the ages of 15 days and 9 weeks. 14C-labelled test substances were injected simultaneously with two reference isotopes, 3H2O and 113mIn-labelled EDTA, into the common carotid artery followed by decapitation 10 s later. There was evidence that a monocarboxylic acid transport system in 15 to 23 day-old rats had a capacity at least six times greater than that present in adult animals. L-Lactate and acetate showed the highest permeability. At all ages there was a constant ratio between L-lactate and (-)D-3-hyroxybutyrate values. D-Glucose permeability increased with age, while that of several amino acids tested was the same in young and adult rats.