Thermally isolated gastrocnemii were stimulated to exhaustion, by rhythmic isotonic (70 N) tetanic contractions, during complete occlusion of blood flow. Enthalpy change (h = work + heat) and work output (w) (kJ/kg) were obtained from records of deep muscle temperature and shortening. The lactate produced (LA, mol/kg) was measured in the outflow after reestablishement of blood flow. The following relationships were obtained: h = 76LA + 1.2, and w = 19.8LA + 0.30. As the energy liberated at exhaustion by alactic energy sources (approximately P and O2 stores) is constant, deltah/deltaLA = 76 (+/- 10.5; S.E.) kJ/mol is the enthalpy change for lactate formation (delta HLA). The neutralization heat was estimated on muscle homogenates at 12kJ/mol, leaving approximately 64 kJ/MOL for deltaH of LA formation proper. The mechanical efficiencies of work related to LA formation (ELA) and of that not related to LA formation (EnonLA) were practically identical (0.25). From these values and from deltaHLA, the enthalpy change of approximately P splitting was estimated in the range of 52--62kJ/mol, depending on the value of the ratio delta approximately P/deltaLA assumed in the calculation.