To examine the specificity of leptin’s hypertrophic action in obesity, the cardiac response of young, i.e. 2 months-old WT (n = 12; body weight, 22 ± 0.9 g), LepRS1138 (n = 9; 34 ± 1.1 g, P < 0.001 vs. WT) and LepRdb/db mice (n = 7; 40 ± 1.3 g; P < 0.001 vs. WT and P < 0.01 vs. LepRS1138) to chronic isoprenaline infusion (20 mg/kg BW per day) was examined. Here, LEP is linked to obesity due to melanocortin 4 receptor deficiency.