the original goal of the present work was to investigate the possibility that endotoxins may act by inactivating the adenine nucleotide translocase system. this approach however to the problem of how endotoxins act had to be abandoned in view of evidence that barnacle muscle fibers have large phosphagen reserves e.g. an arp content of *approx 25 mmoles/kg fiber water and also have a myoplasmic atpmg concentration of *approx. 1.2 mm. it should be recalled however that experiments involving the injection of endotoxin e.g. endotoxin from serrata maresc. (obtained from novotny) showed that the addition of 450 micro to the myoplasm of fibers poisoned with ouabain does sometimes lead to a stimulatory response the same is true of lipid a injection. a different approach viz injection of endotoxin following peak stimulation by acidification of a hco3 -containing medium was adopted. as shown in fig. 1, this, too resulted in a stimulatory response. furthermore, injection of endotoxin following peak stimulation by injected gtp resulted in a stimulatory response. taken together, these results raised the possibility that endotoxin exerts an effect whenever the phosphorylation-dephosphorylation cycle is stepped-up