Anaerobic respiration produces only two ATP from glycolysis. There is no additional ATP produced via the process. Glycolysis is the process of the breaking down of a glucose molecule into two molecules of pyruvate with the release of two ATP. Two NAD are also converted into two NADH in the process. In aerobic respiration, the pyruvate are then further broken down to produce additional ATP but this requires the presence of oxygen. Without oxygen, the NADH needs another mechanism to regenerate NAD for glycolysis. This can happen through lactic acid formation. The pyruvate produced is reduced to lactic acid, thus allowing the NADH to oxidize back to NAD. Lactic acid buildup is what causes muscles to hurt during heavy exercise due to the lack of fast oxygen transport to the muscles to compensate. So there is a net gain of two molecules of ATP from anaerobic respiration.