The short-term influence of catecholamines on surface pH (pHs) and intracellular pH (pHi) of superficial muscle fibres has been investigated in rat soleus in vitro using single-barrelled and double-barrelled glass micro-electrodes. pHs means the pH recorded at the surface of a muscle fibre. All measurements were performed in high-Ca2+ (10 mM) Ringer's solutions. Adrenaline caused an intracellular and surface acidification which increased with concentration in the range 6 X 10(-9)-6 X 10(-6) M. The effect was inhibited by propranolol (10(-5)M) but not by phentolamine (1.5 X 10(-7) M). Noradrenaline and isoproterenol (6 X 10(-6) M) also acidified the intracellular fluid. The relative effect of catecholamines on steady-state pHi was: adrenaline = isoproterenol greater than noradrenaline. Adrenaline (6 X 10(-9)-6 X 10(-6) M) did not accelerate pHi recovery following intracellular acid-loading by NH+4 or CO2. It is concluded that activation of beta-adrenoceptors by catecholamines causes an early intracellular acidosis presumably by enhancing synthesis of metabolic acids. The surface acidification seems at least partly due to non-ionic permeation of sarcolemma by metabolic acids, secondarily inducing accumulation of H+ ions at the cell surface.