Transgenic (TG) mice with cardiac specific 200‐fold overexpression of β₂‐adrenoceptors (β₂‐AR) have a facilitated development of heart failure following thoracic aortic constriction (TAC). We have studied the alterations of intracellular Ca²⁺ transients and myocyte size in wild‐type (WT) and TG mice after TAC. Cardiomyocytes were isolated from mice 9 weeks after TAC or sham operation, and incubated with Fura 2/AM. The Ca²⁺ transients were determined by Spex dual wavelength Spectrometer during electrical stimulation. The cell size was also determined planimetrically. Cells of sham operated TG mice displayed higher systolic Ca²⁺ amplitude than respective WT group (ΔF₃₄₀/F₃₈₀ ratio: 1.05±0.08 vs. 0.63±0.05; P<0.01), a finding in keeping with enhanced ventricular contractility in the TG mice. However, hypertrophied and failing myocytes of TG animals showed a fall in Ca²⁺ transients from sham‐operated control levels and there was no difference between TG and WT groups following TAC. In sham‐operated groups, the cell size of TG mice was significantly bigger than in WT animals (3212±139 vs. 2605±162 μm²; P<0.05). The cell size increased to a similar extent in both groups after TAC (4715±216 vs. 5027±365 μm², P=n.s.). In summary, hypertrophy of cardiomyocytes was present in β₂‐AR TG mice under baseline conditions. A further hypertrophy occurred during pressure overload to an extent similar to that in WT animals. However, the increased intracellular Ca²⁺ transient, seen in sham‐operated TG mice, was no longer detectable following development of severe hypertrophy and heart failure. These findings provide explanation on the lack of hemodynamic benefit in β₂‐AR TG mice subjected to pressure overload.