The presence and functionality of -adrenoceptors (-ARs) were examined in red (RM) and white (WM) muscle 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_d in nM, 0.14 ± 0.03 and 0.18 ± 0.03 for RM and WM, respectively) but with a significant higher number of binding sites in RM compared with WM (B_max in fmol/mg protein, 3.22 ± 0.11 and 2.60 ± 0.13, respectively). Selective and non-selective -adrenergic agonists (-AAs) and antagonists indicated an "atypical" -AR pharmacology. This result may represent a non-mammalian -AR classification or, more likely, the presence of more than one -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 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, 24.67 ± 3.06 and 9.64 ± 3.45, respectively). The agonist-induced increase in cAMP production was blocked by the -adrenergic antagonist propranolol (PROP) while the ACase activator forskolin increased cAMP production by 7- to 14-fold above basal and approximately 3-fold above all -AAs tested. This study demonstrated the presence of "atypical" ₂-ARs on RM and WM membranes of trout, suggesting that ₂-AA may be a tool to enhance protein accretion through this signaling pathway.