In the present study, the density of cell surface #-adrenergic receptors was determined in different skeletal muscles using the hydrophilic ligand [3H]CGP 12177. The density of #-adrenergic receptors was highest in the slow-twitch soleus muscle (32.8-0.9 fmol mg dw-1) and lowest in the fast-twitch glycolytic white gastrocnemius (10.4-0.5 fmol mg dw-1) #-Adrenoceptor density correlated closely with the percentage of type-I fibres (r=0.979; P<0.0001) and inversely with the percentage of type-IIB fibres (r=696; P<0.03). Incubation with isoprenaline (10 µM) for 30 min decreased the density of #-adrenergic receptors in the cell surface from 32.9-0.8 to 19.3-0.7 fmol mg dw-1 in the soleus and from 16.8-1.0 to 12.0-0.7 fmol mg dw-1 in the epitrochlearis. Internalisation appeared rapid (half-time less than 5 min). To study externalisation of #-adrenergic receptors, soleus strips were incubated 30 min with 10 µM isoprenaline and then transferred to buffer without agonist. The first incubation reduced the density to ,50%, the subsequent incubation without agonist increased cell surface receptor density to ,80% of the initial density after 1 h. No further increase was observed over the next 2 h, suggesting that some of the receptors had been degraded. Insulin or contractile activity did not influence rate of externalisation.