<jats:p>The intracellular pH (pHi) of the anterior byssus retractor muscle (ABRM) of Mytilus edulis L. was estimated during rest and during a contraction-catch-relaxation cycle by the distribution of [14C]DMO and by 31P-NMR spectroscopy. The pHi of resting muscles was found to be nearly the same whether it was determined with DMO (7.41±0.06, N=5l) or by 31P-NMR spectroscopy (7.44±0.02, N=5). During catch the pHi was not significantly different from its value at rest. Serotonin (10−5moll−1) induced a rapid relaxation and a significant increase in pHi (DMO: 7.56±0.05, N=5; 31P-NMR: 7.57±0.02, N=5). In resting ABRM serotonin (≥10−6moll−1) also induced an alkalosis. It was shown that cyclic AMP, the second messenger of serotonin, elicited an increase of pHi. Dopamine (10−5moll−1) did not cause an increase in either cyclic AMP levels or pHi. The rates by which tonically contracted ABRM loaded at 0.1 N were stretched were significantly increased by an alkalosis elicited by the addition of 10 or 20mmoll−1 methylamine. Acidosis elicited by lO mmoll−1 DMO caused a significant decrease in the rate of relaxation.</jats:p>