Softening during fruit ripening may be initiated by an increase in proton pumping across the plasma membrane. This would lower the cell wall pH and loosen acid-labile bonds, as well as displacing Ca2+from load-bearing bonds. The enzyme responsible for proton pumping is the plasma membrane H+-ATPase. The activity of this enzyme in nectarine (Prunus persica(L.) Batsch var.nectarina(Ait.) Maxim., cv. Redgold) mesocarp was studied during 4 days of ripening at 20°C after harvest at commercial maturity. Fruit began to produce significant amounts of ethylene (> 1 µlitre/kg per h) from the second day. Flesh firmness began to decline from the third day, but background skin colour did not change until the fourth day. Plasma membranes were extracted from the fruit by phase partitioning. Initial characterisation of the H+-ATPase activity showed that it was Mg2+-dependent, K+-stimulated, and VO43−-sensitive. The Kmfor MgA TP was 0.19 ± 0.02 mM. The Kivalue for Ca2+was estimated at 100 µMfree Ca2+. The in vitro specific activity of the ATPase did not alter significantly during a 4-day period after commercial harvest. There was a slight decline in ATPase activity with increasing ethylene evolution (R2= 0.25, P < 0.05) but no significant correlation between ATPase activity and firmess or skin colour.