The aim of this paper has been to evaluate yields and seed quality of winter oilseed rape depending on the plant density, foliar fertilisation with magnesium, sulphur, boron and Asahi growth biostimulant combined with constant NPK fertilisation. The research was performed in 2006-2008 as a two-factor field experiment. The first factor involved plant density (A – 20, B – 30, C – 40 plants per 1 m2) and the second one comprised fertilisation treatments (n = 7). Foliar fertilisation was applied once or twice. In rape seeds, the following were determined: the content of total nitrogen, content of fat and its fractions. The results show that the concentrations of total nitrogen and fat in rape seeds were significantly dependent on both the plant density and fertilisation. Significantly the highest content of those nutrients occurred after a double application of magnesium sulphate(VI), Solubor and Asahi biostimulant at the plant density of 40 and 30 plants m2, respectively. The composition of fatty acids extracted from seeds of the hybrid oilseed rape cultivar Nelson significantly depended only on fertilisation. The highest content of oleic acid and its increase, as compared with the control, was identified after a single application of magnesium sulphate(VI), Solubor and Asahi biostimulant. The fertilisation significantly decreased the value of the sum of polienic acids C18:2 and C18:3 in rape seeds against the control. It was only after a double application of magnesium sulphate(VI), microelement fertiliser (Solubor) and Asahi biostimulant that the accumulation of these fatty acids remained at the level of concentration reported in the control seeds.