Surface coils with strong coupling for both "P and 'H were used to measure metabolite concentrations by in uiuo N M R spectroscopy. Tissue water was used as an internal concentration reference and the "P and 'H spatial sensitivities of the coils were matched. For such coils, sample loading does not necessarily have a signifierrnt effect on absolute quantitation results. The coils had proportionality constants which were almost independent of loading and the 'H and "P flip angles at the coil centre produced by h e d length pulses were approximately equal over the range of loading conditions encountered in uiuo. For 7 normal infants, of gestational plus postnatal age (GPA) 35 to 37 weeks, the nucleotide triphosphate concentration in the cerebral cortex was 3.7 f 0.6 mmol/L wet tissue (mean f SD). Further studies of normal infants down to 26 weeks GPA indicated that phosphorus metabolite concentrations increased significantly with GPA during this period. Concentrations were often low in the cerebral cortices of birth asphyxiated infants. In order to provide corroboration for the results from neonatal brain, data were acquired also from the resting, unexercised forearm muscles of 6 young adults and the measured adenosine triphosphate Concentration was 6.3 f 0.8 mmol/L wet tissue.