The purpose of this study was to explore mitochondrial activities after oxygen inhalation by 31P magnetic resonance spectroscopy. Six healthy volunteers were studied using a GE 3T scanner. A spin-echo MRS sequence was utilized with a GE service coil. TR was 2000 msec and TE 35 msec with 128 scan averages. Before 31P scan, a 1H MRS pre-scan was performed to obtain shimming values. Exact two 31P scans were performed before and after oxygen inhalation through nasal cannula at a flow rate of 10 L/min. Data were processed offline using the SAGE/IDL software. Peak integral values of inorganic phosphor, phosphocreatine, and ATPs were measured. Peak integral values of inorganic phosphor were decreased from 1.44% to 16.62% (mean+/-SD, 8.2+/-4.60). Peak integral values of phosphocreatine were increased from 1.06% to 8.64% (4.54+/-2.74). Peak integral values of gammaATP were increased from 0.75% to 15.97% (5.94+/-6.10); alphaATP from 1.21% to 16.05% (7.04+/-6.14); betaATP from 1.01% to 7.12% (3.08+/-2.30). Brain mitochondrial activities were increased and more ATPs were produced after oxygen inhalation in healthy volunteers.