1. Chain elongation of fatty acids by extracts of mitochondrial acetone powders from rat liver and pig kidney cortex are similar in their properties. The specific activity of the kidney system is about 30% as compared to the liver system 2. Different incorporation rates [1-14-C] acetate into fatty acids in the presence of NADH as the sole hydrogen donor that were reported in literature can be explained by different extraction methods. 3. In liver the incorporation into the saturated fatty acid, elongated by one C-2 unit, amounts to only 19% with NADH and 60% with NADPH in comparison with the incorporation in presence of both nucleotides. 4. Kinetics of the chain-elongating process favour the enoyl-CoA reductase to be the rate limiting step. 5. Long-chain saturated and unsaturated fatty acyl-CoA derivatives are very poor primers of the chain elongation. 6. Possion and the enoyl-CoA reductase may be the transfer of hydrogen from NADPH to the respiratory chain, and the conservation of reducing equivalents (NADH and NADPH) or acetate units during cellular anoxia.