Mitochondria were isolated from regenerating rat liver 12, 24 and 48 h after partial hepatectomy. The "State 3" and "State 4" respiration were measured in the presence of succinate. The P/O quotient and respiratory control index (RCI) were calculated. The experimental data showed that the partial uncoupling of oxidative phosphorylation in regenerating liver mitochondria occurring in the early period of regeneration is partly due to free fatty acids.