In a closed, static jar experiment implemented in three cycles during the years 2003-2004 at a greenhouse laboratory the competitive influence between spring wheat and spring barley was investigated. It was assessed on the basis of dry mass accumulation in aboveground parts (stems, leaves, heads) and roots as well as crop growth rate. The crops were sown in a mix with equal shares of both components and as single crops on medium soil supplied with standard NPK dose and the dose increased by 50% in relation to the standard one. The number of plants of both crops in mixed cultivation was equal to the sum of plants of both crops cultivated as single crops. Analyses were conducted during the stages of seedling development, tillering, stem elongation, inflorescence emergence and ripening. It was shown that barley had a stronger negative influence on wheat than the other way round. In the mixed crop, as compared to single crop cultivation, the dry mass of wheat was progressively reduced from tillering until inflorescence emergence stage, covering to a similar extent the stems, leaves and heads. Wheat, on the other hand, had a negative influence on the increase of aboveground biomass of barley during tillering, inflorescence emergence and ripening. Until inflorescence emergence stage, the differences between sowing methods appeared for the test samples from jars poorer in NPK, while during ripening they were levelled. The scale of root mass in the mix had similar value to that of the aboveground parts, without the differentiating influence of fertilisation doses. In view of the conducted studies, wheat in the presence of barley was characterised by smaller - than in single crop cultivation - day increases in biomass during the interphases of seedling development-tillering and stem elongation-inflorescence emergence. The biomass accumulation rate for barley cultivated in the mix was subject to gradual slowing from the beginning of vegetation (with the exception of the tillering-stem elongation period). In the case of both cereals the process was more pronounced in the case of more abundant NPK fertilization.