Phospholipid exchange activity has been determined in the supernatant fraction of rat brain from birth through to maturity by measuring the protein-catalysed transfer of total and individual 32P-labelled phospholipids from microsomal membranes to mitochondria, and the transfer of [14C]phosphatidylcholine from liposomes to mitochondria. Transfer activity has also been compared in brain and liver supernatant. Overall phospholipid exchange activity in the brain increased only slightly with age. The activity at birth was 75% of the adult value. However, the transfer of individual phospholipids showed markedly different trends during postnatal brain development. The transfer of phosphatidylinositol (PI) and ethanolamine phospholipids increased postnatally to a maximum at 9 days of age, with lowest values in adult brain. Phosphatidylcholine (PC) transfer increased from 9 days to reach maximum values in the mature brain. The transfer of sphingomyelin was highest immediately after birth. PI transfer activity was higher in brain than liver, while PC and ethanolamine phospholipid transfer activity was higher in liver. The heterogeneity of phospholipid exchange proteins in central nervous system tissue is reflected in the developmental changes in exchange activity towards individual phospholipids. The various exchange proteins appear to have separate induction mechanisms. The presence of exchange-protein activity from birth in the rat indicates the functional importance of phospholipid transport during cell acquisition and membrane proliferation. Activity is not primarily associated with membrane formation such as the formation of the myelin sheath, and therefore is more likely to be involved in the process of phospholipid turnover.