The Siberian hamster is a highly seasonal rodent that undergoes a number of phenotypic adaptations for winter survival that culminates in bouts of daily torpor. During torpor bouts body temperature may fall by 20^oC which would ordinarily induce fatal ventricular arrhythmias in non-torpid species such as man. Sex hormones have a strong influence on these adaptive changes. The purpose of the present study was twofold; i) to determine the mechanism responsible for changes in cardiac intracellular calcium homeostasis occurring in response to shortened day-length and ii) determine if these changes require the presence of sex hormones. All procedures comply with current UK legislation. Under isoflurane anaesthesia hamsters of either sex were gonadectomised. Two weeks after gonadectomy animals were either maintained on a long photoperiod (LP, 16h light:8h dark) or short photoperiod (SP, 8h light:16h dark) regime for 12 weeks. Animals were then killed in mid-light phase by cervical dislocation and single ventricular myocytes isolated using a collagenase and protease digestion technique. Cells were voltage clamped at 37^oC using the perforated patch technique and stimulated at physiologically relevant rates for torpid and normothermic animals (1-6Hz). Changes in intracellular calcium concentration were measured using Fluo-3AM. Data are expressed as mean ± SEM and assessed for differences as indicated. Following voltage clamp stimulation at 2Hz the amplitude of the systolic calcium transient was increased in SP animals (585±89 versus 244±42 nmol/l, n = 9 each group, p<0.05 Mann-Whitney U test). There was no change in peak L-type calcium current density in response to altered photoperiod. However, across all frequencies sarcoplasmic reticulum (SR) calcium content was increased (p<0.05; Two-Way ANOVA, effect for photoperiod). There was also an increase in the SR dependent rate of Ca uptake in SP cells (11.7±1.2 versus 7.2±0.8 s^-1, n = 7-9 cells, p<0.05, Students t-test). Despite these changes in SR mediated calcium uptake and systolic calcium transient amplitude no changes in mRNA or protein levels for the SR calcium ATPase, phospholamban or calsequestrin were found. In conclusion we observed marked changes in intracellular calcium homeostasis between short and long day acclimated Siberian hamsters. Importantly these changes do not depend on the presence of sex hormones. We speculate that the increase in SR calcium content and thus calcium transient amplitude observed in SP animals may occur due to alterations in the phosphorylation status of phospholamban and that the increased SR mediated calcium uptake is a key step in protecting against cardiac arrhythmias during torpor.