The strength of cardiac contraction is normally consistent from beat to beat. However, during heart failure mechanical alternans (a beat to beat alternation in the strength of contraction) may develop. Mechanical alternans arise from an alternation of the amplitude of the systolic Ca transient. Previously we have shown in cardiomyocytes that alternans can be induced by stimulating with very small depolarising pulses such that Ca entry via ICa is reduced (e.g. Díaz et al., 2002; Díaz et al., 2004). This allows normal Ca-induced Ca release (CICR) to take place at only a few points in the cell. This fragmentation of the Ca release profile increases the probability of alternans occurring. During metabolic inhibition, reduced [ATP]i shortens the action potential and intracellular acidosis and increased Mg reduce the open probability (Po) of the RyR. Previous work (using cells loaded with high concentrations of Ca buffers) has shown that metabolic inhibition induces a dyssynchronous SR Ca release pattern by reducing the number of SR release sites recruited (Fukumoto et al., 2005;Chantawansri et al., 2008). Our aim was to determine if metabolic inhibition increases susceptibility to mechanical alternans in isolated rat cardiac myocytes when no changes to the Ca buffering capacity of the cell have been made as this would limit the ability of the cell to sustain propagating CICR. Metabolic inhibition was induced by the application of solution containing 2mM CN and the exclusion of glucose. [Ca2+]i was measured using Fluo-3 AM. The results show that pulses which were too large to produce alternans in control did so in metabolic inhibition. This was not due to a decrease of ICa, as at the point where alternans began the mean current was 54.7 ± 16.6 pA (mean ± s.e.m.) in control and 108 ± 43.5 pA in metabolic inhibition. When alternans was present in control, it became larger in metabolic inhibition. A convenient measure of the degree of alternans is provided by the ratio of the Ca efflux on Na-Ca exchange during a large transient compared to that during a small transient. This ratio was maximal in control at 2.4 ± 0.9 at 18 mV depolarisation and increased to 5.3 ± 1.3 at maximum in metabolic inhibition (n=5; p=0.03) with depolarisations of 22mV. This increased alternans amplitude in metabolic inhibition may be due to the increased SR Ca content we have previously reported. This would make the SR more likely to respond to a small trigger ICa giving either more release sites responding or faster propagation of mini-waves responsible for this type of alternans