A wide range of biological processes are known to be controlled by changes in photoperiod. In mammals an extreme example of such a process is the state of torpor where body temperature, metabolic rate and heart rate decrease markedly and provide a mechanism whereby animals are able to survive the intense cold of winter. The hearts of hibernating animals not only have to adapt to these extremes of physiology but unlike the hearts of non-hibernating species, such as man, have also been shown to be remarkably resistant to the development of ventricular fibrillation (Duker et al., 1983). The purpose of the present work was to determine the nature, if any, of the changes in cardiac excitation contraction coupling that occur in response to long-term changes in photoperiod. All experiments were performed on single cardiac myocytes isolated from the hearts of the Djungarian hamster. Animals were either maintained on a short day (SD; 6h:18h light:dark) or long day (LD; 18h:6h light:dark) cycle for 12weeks prior to humane sacrifice. During this time SD animals exhibited pelage, weight loss and bouts of spontaneous torpor. Changes in intracellular calcium concentration were measured using Fluo-3AM whilst the perforated patch clamp technique was used to record action potentials and membrane currents. All cellular experiments were performed at 37^oC. The amplitude of the systolic calcium transients from myocytes of SD animals were larger than those from LD animals at all frequencies of stimulation tested (4-8Hz, n=7-16 cells, P<0.05 by Repeated Measures ANOVA) e.g. at 6Hz (mean±s.e.m) 369±46 vs. 193±27 nmol.L^-1 in SD and LD animals respectively. Whilst diastolic calcium concentration increased with increasing stimulation frequency, there was no difference between SD and LD animals. Changes in action potential duration were not responsible for the inotropic effect of SD adaptation as this parameter was the same in SD and LD animals. However, measurements of sarcoplasmic reticulum calcium content revealed that SD animals had a greater releasable store of calcium than LD animals e.g. following action potential stimulation at 6Hz the sarcoplasmic reticulum calcium content increased from 139±9 to 196±11 μmol.L^-1 (n=6-11 cells, P<0.05). In conclusion, in preparation for torpor cardiac excitation contraction coupling undergoes marked changes. How such changes confer resistance to ventricular fibrillation and the molecular mechanisms responsible for such changes require elucidation. All procedures accord to UK legislation