The mdr1a isoform of P-glycoprotein (Pgp) is an integral plasma membrane efflux pump expressed in adult brain capillary endothelial cells and astrocytes of the blood-brain barrier. We determined the developmental pattern of Pgp expression in brain tissue at embryonic day 16 (E16), day of life 0 (D0), day of life 7 (D7), day of life 21 (D21), and adults (Ad). The relative expression of Pgp mRNA and protein was indexed as a percent (mean ± SEM) of D0 levels. Pgp mRNA levels increased significantly (p < 0.01) with maturation (E16: 75 ± 8%; D21: 303 ± 37%, and Ad: 1,160 ± 120%). Similarly, Pgp protein expression was observed at E16 and increased significantly (p < 0.01) during development (E16: 52 ± 8%; D7: 187 ± 23%; D21: 440 ± 48%, and Ad: 441 ± 56%). This developmental pattern of enhanced blood-brain barrier Pgp expression with maturation was confirmed by immunohistochemistry. We conclude that (i) Pgp expression in mouse brain is limited during late embryogenesis and the newborn period; (ii) Pgp expression increases markedly with postnatal maturation, and (iii) by D21 brain Pgp protein expression approximates adult levels.