Based on the citrate model of Lobit and colleagues and measured data, a new model, which is able to reproduce the variation over time of citrate concentration in two peach cultivars, has been proposed. As in the original one, the new model calculates the rate of citrate synthesis or degradation as the product of a 'synthesis potential' and an 'efficiency level'. While in the old model the 'efficiency level' was a simple linear function of temperature and respiration, in the new one its relationship with respiration is accounted for by a coefficient that decreases throughout fruit development. The differences in model parameters between the two cultivars were investigated: late-maturing cv. Suncrest had significantly lower citrate synthesis potential than mid-maturing cv. Fidelia. The responses of citrate concentration to model parameters, temperature, fruit respiration, and growth curves were studied. The most important parameter in the new model, k(4,2), represented the date when the relationship between respiration and 'efficiency level' changed from positive to negative. Raising mean temperature increased the citrate concentration at the beginning and decreased it near maturity for cv. Suncrest, while citrate concentration increased throughout fruit development and more strongly for cv. Fidelia. An increase in the mesocarp dry weight increased both fruit respiration and citrate concentration at the beginning of fruit development, while near maturity it increased fruit respiration but decreased citrate concentration. The model was also able to reproduce the effect of assimilate supply (leaf:fruit ratio). Further potential uses of the model were discussed