1. This study examines beta(1)-, beta(2)- and beta(3)-adrenoceptor (AR)-mediated responses, mRNA levels and radioligand binding in ileum from beta(3)-AR knock-out (-/-) (KO) and wild type (+/+) (FVB) mice. 2. In KO and FVB mice, SR59230A (100 nM) (beta(3)-AR antagonist) antagonized responses to (-)-isoprenaline in both KO and FVB mice. (-)-Isoprenaline mediated relaxation of ileum was antagonized weakly by ICI118551 (100 nM) (beta(2)-AR antagonist). Responses to (-)-isoprenaline were more strongly antagonized by CGP20712A (100 nM) (beta(1)-AR antagonist), propranolol (1 microM) (beta(1)-/beta(2)-AR antagonist), carvedilol (100 nM) (non-specific beta-AR antagonist), and CGP12177A (100 nM) (beta(1)-/beta(2)-AR antagonist) in ileum from KO than in FVB mice. 3. Responses to CL316243 (beta(3)-AR agonist) in ileum from FVB mice were antagonized by SR59230A (100 nM) but not by propranolol (1 microM) or carvedilol (100 nM). CL316243 was ineffective in relaxing ileum from KO mice. 4. CGP12177A had no agonist actions in ileum from either KO or FVB mice. 5. beta(1)-AR mRNA levels were increased 3 fold in ileum from KO compared to FVB mice. This was associated with an increased maximum number of beta(1)-/beta(2)-AR binding sites (B(max)). beta(2)-AR mRNA levels were unaffected while no beta(3)-AR mRNA was detected in KO mice. 6. In mouse ileum, beta(3)-ARs and to a lesser extent beta(1)-ARs are the predominant adrenoceptor subtypes mediating relaxation in ileum from FVB mice. In KO mice beta(1)-ARs functionally compensate for the lack of beta(3)-ARs, and this is associated with increased beta(1)-AR mRNA and levels of binding.