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The presence and functionality of β-adrenoceptors (β-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 ± 0.03 and 0.18 ± 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 ± 0.11 and 2.60 ± 0.13. respectively). Selective and nonselective β-adrenergic agonists (β-AAs) and antagonists indicated an atypical β-AR pharmacology. This result may represent a nonmammalian β-AR classification or, more likely, the presence of more than one β-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 β[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. · 10 min[sup -1]: 24.67 ± 3.06 and 9.64 ± 3.45, respectively). The agonist-induced increase in cAMP production was blocked by the β-adrenergic antagonist propranolol, while the ACase activator forskolin increased cAMP production by 7- to 14-fold above basal and ∼3-fold above all β-AAs tested. This study demonstrated the presence of atypical β[sub 2]-ARs on RM and WM membranes of trout, suggesting that β[sub 2]-AAs may be a tool to enhance protein accretion through this signaling pathway. [ABSTRACT FROM AUTHOR]