The nutritive compounds of the onion are influenced by genetic as well environmental factors. Due to its shallow poorly-developed root system, onion has a low potential of using nutrients from soil. Usually mineral fertilisation increasing the yield and change the chemical composition in vegetables, and it is the chemical composition which determines the nutritive value of onion. Over 2003–2006 at Korytowo, about 30 km away from Bydgoszcz, northwards, 2-factor field experiments were set up in ‘split-plot’, in three reps. The aim of the study has been to determine the effect of the nitrogen rate (0, 60, 120, 180 kg N·ha–1) on the content of dry matter, vitamin C and monosaccharides and total sugar in onion of two cultivars: Kutnowska and Efekt. Increasing nitrogen fertilisation has resulted in a decrease in the content of dry matter in the bulbs of the onions under study. The ‘Kutnowska’ showed a higher content of total sugar and, at the same time, a lower content of monosaccharides than ‘Efekt’. ‘Efekt’ reacted with an increase in the content of vitamin C and monosaccharides as affected by higher fertilisation rates. A different reaction was noted in ‘Kutnowska’. The lower the dry matter content in onion bulbs – the greater content of total sugars and less content of monosaccharides.