The characteristics of hepatic β 2 -adrenoceptors (AR) were examined in rainbow trout (Oncorhynchus mykiss) chased once per day to exhaustion for up to 7 days or fed the repartitioning agents clenbuterol (CLEN) or ractopamine (RACT) that function in mammals as β-agonists. A one-day chase and feeding the CLEN for 37 days resulted in a significant 27% and 33% decrease, respectively, in the number of CGP-binding sites (B m a x ) with no significant change in affinity (K d ) of hepatic β 2 -ARs. Despite the significant decrease in β 2 -AR numbers with CLEN feeding, no significant differences were found for either β 2 -AR mRNA levels or adenylyl cyclase (ACase) activities. In addition, CLEN displayed only partial agonist activities as it was found to be more effective at blocking isoproterenol-stimulated cAMP production in isolated hepatocytes than stimulating cAMP production. The small affects of RACT may be related to its low active stereoisomer content and low affinity for the trout β 2 -AR. Agonist regulation of the trout hepatic β 2 -ARs may involve down-regulation of the receptors without affecting responsiveness.