The mechanism of DCCD inhibition of ATP synthetase, and the components of the mitochondrial membrane with which DCCD interacts have been investigated. It has been shown that DCCD inhibits ATP-dependant reactions in rat liver mitochondria and that 14C-DCCD is covalently bound to a proteolipid with a molecular weight of 10,000 daltons. This proteolipid may be synthesised on mitoribosomes. The role of the membrane in the mechanism of inhibition of the Mg2+ ATPase has been demonstrated by perturbation of the membrane with diethyl ether, such that inhibitor sensitivity, but not enzymic activity, is destroyed. A series of oligomycin resistant mutants of Saccharomyces cerevisiae have been found to be cross- resistant to DCCD. An oligomycin (DCCD) sensitive ATPase has been prepared from the mitochondria of these mutants, and the mutant enzyme shown to possess a lowered sensitivity to DCCD. It is suggested that one of four subunits of the ATPase is the site of action of DCCD. It has been found that the smallest subunit will bind 14C DCCD when the mitochondrial membrane is depleted of F1 and OSCP. This subunit has been extensively purified from mitochondria of both parental and oligomycin (DCCD) resistant strains of S. cerevisiae and the mutant peptide has been shown to differ in composition from that derived from the parental strain.