To investigate whether STAT3 may be important for cardioprotection during β-AR treatment independent of pregnancy and pregnancy hormones, male CKO mice were subjected to Iso treatment (10–30 mg/kg/day) for 2 weeks, which led to a dose-dependent increase in mortality compared with WT siblings (Figure 1C). In CKO mice, Iso treatment induced LV dysfunction and increased LV dilatation with increased heart/body weight ratios (Table 2; Supplementary material online, Figure S1) and cardiomyocyte lengths (CML: +33 ± 14%, *P =0.0001 vs. CKO NaCl; *P indicates two-way ANOVA) with no significant increase in cardiomyocyte cross-sectional area (CSA: +32 ± 28%, *P =0.0861 vs. CKO NaCl). In WT and heterozygous (aMHC-Cre^tg/+; Stat3^fl/+) mice LV function remained compensated (Table 2; Supplementary material online, Figures S1 and S2). Iso-treated WT mice displayed ventricular hypertrophy (Supplementary material online, Figure S1) and increased CSA (+50 ± 38%, *P =0.0101 vs. WT NaCl) with no change in CML (+9 ± 12%, *P =0.3317 vs. WT NaCl). Blood pressure was similar in both genotypes (Supplementary material online, Figure S1). Similar to males, nulli-pari female CKOs developed LV dysfunction in response to Iso (fractional shortening: CKO, 22 ± 6%; WT, 44 ± 5%; ^#P <0.0001; ^#P indicates two-tailed Student's t-test). The β₁-selective blocker metoprolol largely attenuated Iso-induced adverse effects in CKO mice, while bromocriptine had no effect (Table 2). While Iso stimulation enhanced phosphorylation of troponin I (TnI) at the protein kinase A (PKA) and C (PKC) sites similarly in CKO and WT mice (Figure 1D and E), a more pronounced increase in cardiac fibrosis, cardiomyocyte necrosis, and inflammation and higher expression of atrial and brain natriuretic peptides (BNP) were observed in CKO compared with WT mice (Figure 1F–K; Supplementary material online, Figure S1).