The turnover of phosphorus compounds in crab muscle fibres.

1. The exchange of 32P injected into single crab muscle fibres as either orthophosphate or ATP has been studied. The results have been analysed in terms of a simple kinetic model. 2. The exchange of 32P between ATP and arginine phosphate is considerably faster than that between ATP and orthophosphate during rest and during contraction in caffeine. 3. The rate constants obtained at rest after the injection of either orthophosphate or ATP labelled with 32P were of the same order. They were also of the same order as those which have been obtained previously for squid axons. 4. The exchange during caffeine contraction of 32P unjected as orthophosphate gave rate constants which were consistent with an increased rate of splitting of ATP and with the rate constants needed to account for the observed net formation of orthophosphate. The results obtained after the injection of 32P-labelled ATP during caffeine contraction were less clear cut.