The results reported herein represent the first step in diagnosing the nutrient status of citrus trees by means of foliar analysis, for the average conditions of the State of São Paulo. A study was made concerning the seasonal variation in nitrogen, phosphorus, potassium, calcium and magnesium levels of spring-cycle Baianinha orange leaves. The trees were budded on sweet orange rootstock (Citrus sinensis Osbeck) in a foctorial fertilizer experiment started in 1947. The mineral composition changed with the age of leaves. Generally the N, P, K, and Mg contents decreased, while Ca increased fast in the young leaves and remained without great variation until the end of the cycle. Leaves 3 to 6 and 10 months old are suitable for sampling if referred to respective standards. Nitrogen and phosphorus levels in leaves increased linearly with fertilizer application, while potassium increases were due only to o single dosis. Application of fertilizer containing nitrogen shows o tendency to reduce the levels of P and K, and to increase Mg in the leaves. The simple superphosphate applied decreased the levels of N and K, and increased Ca in the leoves. There was a decline in Ca and Mg contents of leaves with potassium fertilizer application. The following negative correlations between leaf nutrients were found to be significant: 1) nitrogen and phosphorus; 2) nitrogen and calcium; 3) potassium and calcium; 4) potassium and magnesium.