The contribution of anaerobic metabolism in normoxic and hypoxic anterior byssus retractor muscles (ABRM) to ATP production during tonic and repeated phasic contractions under isotonic conditions was estimated by analysing changes in the levels of ATP, phospho-l-arginine, l-arginine, octopine, alanopine, strombine, l- and d-lactate, succinate, l-aspartate and l- and d-alanine. 1. During tonic contraction under normoxia, elicited by the application of acetylcholine and subsequent washout, muscles depleted the phospho-l-arginine store by 3 μmol·g wet wt-1. The phosphagen was replenished during the maintenance of catch. Formation of anaerobic end products was not observed. The energy demand during repeated phasic contractions elicited in aerated media containing serotonin was not only indicated by a continuous breakdown of the phosphagen but also by decreasing ATP levels and the accumulation of octopine. 2. The energy demand during tonic contraction under hypoxic conditions {Mathematical expression} was indicated by a breakdown of phosphagen as well as by ATP depletion. During catch, however, ATP was rapidly replenished whereas restoration of the phosphagen pool to control levels was delayed. Octopine and succinate were accumulated during the initial 30 min of catch and remained constant thereafter. The additional anaerobic ATP turnover (ATPt) during the active phase of isotonic contraction was found to be 4.9±1.4 μmol·g wet wt-1 accounting for the work done. An additional ATP turn-over during subsequent catch was not detectable. 3. Hypoxia {Mathematical expression} did not change the contractility of ABRM during phasic contractions. Energy was provided by means of phosphagen and ATP depletion, by octopine and succinate formation and, in addition, some accumulation of alanopine. © 1989 Springer-Verlag.