The 'calcium paradox', i.e. irreversible loss of mechanical and electrical activity after a few minutes Ca2+-free perfusion and subsequent reperfusion with Ca2+-containing medium, was originally demonstrated in rat heart and later in other homoiotherms. It was absent in poikilotherms. The present paper describes the effect of Ca2+-free superfusion on human myocardium at normo- and hypothermia using isometrically mounted, stimulated strips from auricular tissue excised during cardiac surgery. The experiments (including equilibration period) were performed at 37 degrees C, 30 degrees or 25 degrees C. When Ca2+-free and EGTA-containing solution was introduced the force decline started immediately. After 10 min Ca2+-containing medium was reintroduced. Force recovery within 40 min was at 37 degrees C 16% and at 30 degrees C 28% of initial force value. Force recovery in 25 degrees C group was substantially better. Contracture which occurred when Ca2+-containing solution was reintroduced was very strong at 37 degrees C (+ 102%) and weaker at lower temperatures (30 degrees C +48% and 25 degrees C +17%). The extent of loss of mechanical activity and of contracture following the period of Ca2+-free superfusion give evidence that calcium paradox can be provoked in human myocardium and can be prevented by hypothermia.