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The presence and functionality of [beta]-adrenoceptors ([beta]-ARs) were examined in red (RM) and white muscle (WM) membranes isolated from the rainbow trout Oncorhynchus mykiss. Specific binding assays revealed the presence of a single class of binding sites with similar affinities in both muscle types ([K.sub.d] in nM: 0.14 [+ or -] 0.03 and 0.18 [+ or -] 0.03 for RM and WM, respectively) but with a significantly higher number of binding sites in RM compared with WM ([B.sub.max] in fmol/mg protein: 3.22 [+ or -] 0.11 and 2.60 [+ or -] 0.13, respectively). Selective and nonselective [beta]-adrenergic agonists ([beta]-AAs) and antagonists indicated an atypical [beta]-AR pharmacology. This result may represent a nonmammalian [beta]-AR classification or, more likely, the presence of more than one [beta]-AR subtype in trout muscles with similar affinities that could not be kinetically resolved. Adenylyl cyclase (ACase) assays showed a dose-dependent increase in cAMP production as concentrations of [[beta].sub.2]-AAs increased in both muscle membranes with significantly higher basal cAMP production in RM compared with WM (cAMP production in pmol cAMP*mg [protein.sup.-1]*10 [min.sup.-1]: 24.67 [+ or -] 3.06 and 9.64 [+ or -] 3.45, respectively). The agonist-induced increase in cAMP production was blocked by the [beta]-adrenergic antagonist propranolol, while the ACase activator forskolin increased cAMP production by 7- to 14-fold above basal and ~3-fold above all [beta]-AAs tested. This study demonstrated the presence of atypical [[beta].sub.2]-ARs on RM and WM membranes of trout, suggesting that [[beta].sub.2]-AAs may be a tool to enhance protein accretion through this signaling pathway. [beta]-adrenoceptor; adenylyl cyclase; adenosine 3',5'-cyclic monophosphate; Oncorhynchus mykiss