Background. Previous work has shown significant swelling of isolated rabbit myocytes exposed to cold hyperkalemic cardioplegia; however, the effect of warm hyperkalemic cardioplegia on myocyte volume is unknown. This study examined the effect of warm hyperkalemic cardioplegia (St. Thomas' solution) on myocyte volume. Methods. Myocytes were enzymatically isolated and placed on an inverted video microscope. Tyrode's solution (37°C) was infused for 10 minutes to establish baseline cell volumes. Subsequently, either the control Tyrode's or St. Thomas' was infused either at 37°C and 9°C respectively (n = 5 for all groups) for 20 minutes, followed by a 30- minute reperfusion with 37°C Tyrode's. Cell volume was determined from cell images captured every 5 minutes. Results. Myocyte swelling occurred rapidly on exposure to cold St. Thomas' solution to a maximum of 9.8 ± 2.1% (p < 0.001). In contrast, myocytes exposed to warm cardioplegia did not show any volume changes during exposure to cardioplegia. However, upon reexposure to Tyrode's, these cells showed shrinkage below their baseline volume (p < 0.001). Conclusions. The cell swelling associated with hypothermic cardioplegia is prevented by normothermic infusion. (C) 2000 by The Society of Thoracic Surgeons.