The changes in f. wt, d. wt, nitrogen, potassium, calcium, phosphorus, magnesium and carbon of the seventh leaf of a tomato plant were measured at seven occasions from 10 days to 30 days after leaf emergence. Measurements of CO2 exchange by the leaf during both light (70 W m−2, 7 h) and dark (17 h) periods and the change in carbon content over these two periods enabled a carbon balance to be constructed on these seven occasions. Changes in the sugars and starch contents of the leaf over these two periods at each occasion were measured. With the exception of calcium the rates of accumulation of all substances increased to their maxima when the leaf was 22–24-days-old. Carbon fixation per unit f. wt. increased to a maximum when the leaf was 16-days-old. In a 10-day-old leaf the rate of carbon fixation was already four-fifths of maximum and one-quarter more than that at 30 days. The rate of night respiratory loss of carbon per unit fresh weight decreased as the leaf expanded. In a 10-day-old leaf, the amount of carbon lost by night respiration accounted for one-quarter of that fixed in the same day. This fraction fell to one-tenth when the leaf was 22-days-old and remained constant thereafter. The amount of carbon being imported to the leaflets of a 10-day-old leaf was less than one-quarter of that accumulated in 1 day. Thus, the contribution of the imported carbon to the leaf growth up to this stage is relatively small. The transition of the seventh leaf from being a net importer to being a net exporter occurred when the leaf was 13-days-old. The sucrose content per unit f. wt was higher in the younger than in the older leaves and was not correlated to the transition from net import to net export. The accumulation and breakdown of starch in a leaf were related not only to the growth of the leaf but also to the development of the whole plant.