European Union regulations introduced in 1997 to limit the nitrate concentrations in some vegetable crops may create problems for growers of protected crops in northern Europe where leafy vegetables can accumulate high nitrate levels in leaf tissues. This paper examines the effect of N supply on two butterhead lettuce crops (Lactuca sativa L.) by combined experimental work and modelling in order to improve our understanding of the underlying processes of N use and plant growth. Transplanted lettuce crops were grown in winter and summer in plots of the same soil. Ammonium nitrate fertiliser was applied immediately before planting at rates from 0 to over 200 kg of N/ha and measurements of crop growth and organic-N concentration made at intervals until maturity. A model was built that predicts the time course of dry weight and organic-N content of the crop. This has been designed with a user-friendly interactive interface. The interface provides a graphical comparison of the predictions with the experimental data, allowing the effects of different N fertiliser levels to be evaluated quickly. The driving variables are: air temperature, solar radiation and mineral N content of the soil. The results showed that the optimum N fertiliser rates were greater for the summer crop suggesting that more mineral N is needed to meet its higher yield and to satisfy the larger instantaneous demand created by its faster growth rate. Preliminary tests have shown that the model is capable of providing an accurate description of response to N supply from independent data in experiments at three sites (two on commercial nurseries). Simulations showed that at planting, soil mineral-N in the top 20 cm has a major effect on growth and determines the amount of N-fertiliser required. Thus, the model can be used to optimise the N-supply to ensure good yields without excessive N accumulation in soil-grown greenhouse lettuce crops.