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Cotton plants, Gossypium hirsutum L. cv. Antares, were grown in nutrient solution, under three levels of nitrogen (N) supply, in order to assess their effects on growth and yield, and to evaluate the N status by the following methods: visual symptoms, changes in the leaf ultrastructure, direct and indirect determinations of leaf chlorophyll (SPAD readings); NO3-N in the petiole both, by spot test in vivo and by quantitative determination; total N in the leaf blade and nitrate reductase activity (NO3 R) in the same; total N in the seeds. Low N level in the solution drastically reduced vegetative growth components, number of bolls and seed cotton and fiber yields. Nitrogen deficiency caused yellowing of the leaves beginning in the older ones. The main changes in the ultrastructure of the N deficient leaves were the disruption of the chloroplast membrane and the accumulation of starch granules and lipid globules. The semiquantitative assessment of nitrate in the petiole was unable to differentiate between the upper levels of nitrogen supply, this being also the case for seed N. Nitrogen status was well correlated both with the quantitative determination of NO3-N and with the NO3 R activity. Both direct and indirect determinations of leaf chlorophyll (SPAD readings) were able to reflect the N status, showing significant correlation with yield. [ABSTRACT FROM AUTHOR]