Broccoli (Brassica oleraceavaritalicacv. Premium Crop) plants were germinated in soil, transferred to vermiculite three weeks later and grown in the greenhouse, supplied continuously with boron concentrations ranging from 0.0 to 12.5 mg L‐1. At commercial maturity, the partitioning of nitrogen into soluble (nitrate, ammonium, amino acids) and insoluble components of the foliage (young and old leaves) and the florets was investigated. Both boron deficiency and toxicity increased the per cent soluble nitrogen, particularly as nitrate. Boron toxicity, but not deficiency, consistently affected the concentration and relative composition of amino acids. The actual nitrogen partitioning, including the relative amino acid composition, was dependent upon the developmental stage and of the plant organs, and whether boron was present in deficient or toxic levels. Together with information about the composition of transport fluids from a previous study, the data are interpreted as evidence for a reduction in the availability of carbon under conditions of boron stress.