A combined morphological and biochemical investigation of mitochondria from developing and rapidly growing tissues ( tumors, fetal, and very early neonatal rat liver) revealed mitochondria which were deficient in respiratory control, showed no valinomycin induced K + accumulation or spontaneous Ca+ + uptake, and were unable to undergo a swelling-contraction cycle. Electron microscopic examination of fetal and neonatal livers and a mammary tumor revealed mitochondria which differed from controls with respect to matrix density and ability to undergo reversible structural changes. The importance of isolation and assay media in interpretation of results is emphasized.