Flash height recorded following the injection of firefly into the external calibration medium depends on the concentration of K-glutamate used, e.g. 120 mM K glutamate reduces flash height by approximately 20 percent. 2. Suspending the fibre in air instead of artificial sea water (ASW) or replacement of NaCl in the bathing medium with Na-glutamate fails to alter flash height. 3. Firefly preparations from DuPont, Packard and SAI give similar myoplasmic ATPMg values viz. 1.1 mM. 4. Analysis of 36 fibres shows the following: myoplasmic ATP = 1.03 +/- 0.06 mM; total ATP (firefly method) = 5.26 +/- 0.12 mmol/kg water; total ATP (enzymic fluorimetry) = 6.27 +/- 0.13 mmol/kg water and ArP = 20.76 +/- 0.59 mmol/kg water. 5. Measurement of ATPMg in samples of myoplasmic aspirate gives a value that is greater than that obtained in situ. 6. Iodoacetate, whether applied externally or internally, reduces resting luminescence in a dose-dependent manner. It also reduces myoplasmic ATP and total ATP. 7. 2-Deoxy-d-glucose fails to reduce myoplasmic ATP but reduces total ATP. 8. Diethylpyrocarbonate, whether applied externally or internally, reduces myoplasmic ATP. It also causes a slow decline in ArP but little change in total ATP. 9. Injection of L-arginine causes a fall in resting luminescence in some fibres while in others it causes a prompt transitory rise. Injection of L-arginine also causes a fall in total ATP. 10. Collectively, these results suggest that the immediate buffering system in the myoplasm is ArP and that ATP supplied by glycolysis lies in a compartment, presumably the interfibrillar space, which is inaccessible to injected firefly.
