The bioavailability of certain metal ions in grape must has been shown to be an important factor in governing fermentation performance by wine yeasts. Elevating levels of external magnesium by supplementing growth media, or increasing intracellular concentrations of magnesium in yeast by cellular "pre-conditioning", resulted in a stimulation of yeast growth, sugar consumption rates and ethanol productivity. Elevation of calcium levels, however, tended to result in suppression of fermentation, presumably by interfering with the cellular uptake of magnesium, since the two metals are known to act antagonistically in biochemical functions. Maintenance of high magnesium:calcium concentration ratios, which are normally low in grape must, may have served to alleviate antagonism of essential magnesium-dependent yeast functions by calcium. Wine produced following fermentation with altered levels of magnesium and calcium exhibited different organoleptic profiles and implications for wine yeast physiology and wine making are discussed.