To determine the effect of i.v. indomethacin on cerebral intracellular oxygenation, as judged by changes in the concentration of oxidized cytochrome oxidase, studies were performed using near-infrared spectroscopy on 15 infants of 23 to 29 wk gestation, aged 8 to 30 d, who required the drug for closure of a patent ductus arteriosus. Indomethacin caused significant (p < 0.01) falls in cerebral blood flow, oxygen delivery, and blood volume and its reactivity to changes in arterial carbon dioxide tension, as previously reported. In 11 of the 15 infants, significant (p < 0.01) falls of 0.18 to 0.92 mumol.L-1 in the concentration of oxidized cytochrome oxidase were observed. Parallel experiments on synaptosomes derived from rat brain showed that indomethacin was most unlikely to have had a direct effect on mitochondrial respiration. These results are consistent with the hypothesis that the observed falls in the concentration of oxidized cytochrome oxidase resulted from hemodynamic perturbations that reduced cerebral intracellular oxygenation.