The biochemical changes associated with brain maturation during the first 28 days postnatal were investigated utilizing proton and phosphorus-31 nuclear magnetic resonance spectroscopy in rat pups in vivo. Phosphocreatine was found to increase linearly during this period of development. Phosphomonoester was high at birth, peaked around the 10th day postnatal, and declined thereafter. N-Acetyl-aspartate was low at birth, increased in an approximately linear fashion, and reached adult levels by about Day 28 postnatal. Choline was high at birth and declined thereafter. Taurine, a sulfur amino acid abundant in fetal brain, was also present in high levels on the first day postnatal.