Significant activation of AMP deaminase in fast-twitch muscle leads to a loss of ATP and accumulation of NH4 and IMP. Although this occurs during severe metabolic stress caused by intense contraction conditions, the process is probably influenced by the muscle's capacity for aerobic metabolism. We evaluated this possibility during moderately intense (5 Hz) contraction conditions in situ by following the time course of NH4 and IMP accumulation in fast-twitch, low-oxidative white (FTW) and fast-twitch, high-oxidative red (FTR) muscle of the rat. A high rate of IMP formation, resulting in a 50% loss of ATP content, occurred in normal FTW, but not FTR muscle, during contractions when blood flow was intact. Eliminating blood flow prior to contractions, however, removed the distinction between the FTR and FTW muscle. The FTR fiber section now produced a high IMP content and a stoichiometric loss of ATP. Thus the ability of the FTR fiber to sustain this contraction effort without an ATP loss is due to its greater functional capacity for aerobic metabolism. The FTW muscle section of trained animals exhibited a reduced accumulation of IMP and a smaller loss of ATP during the same 5-Hz stimulation. The mitochondrial content and peak blood flow of this FTW fiber section is increased by training. Thus it is probable that the cellular conditions leading to a significant accumulation of IMP in fast-twitch muscle are determined by the metabolic stress established by the contraction effort, relative to the muscle fiber's functional capacity for aerobic metabolism.