The determination of grain weight, defined as the product between the rate and duration of grain filling (fig 2), was analysed in field experiments with genotypes, sowing times, water and nitrogen supplies (tables I, II, III). The functional relationships between principal crop characteristics used in data analysis are presented in fig 3. In natural environments, the filling rate is the main factor determining grain weight (figs 4 and 5). It is controlled by the number of kernels per m2 (fig 7). Duration and rate are negatively correlated (fig 6). The duration does not compensate for the reduction in the weight of a grain produced by a smaller rate of filling (fig 5). It is controlled by the rate of filling together with environmental factors. Accordingly, the number of kenels/m2 is a principal factor controlling parameters of the grain filling period and the adaptability to environment. It also plays an active part in the determination of potential yield, because of change in source-sink proportion (fig 8). The relations rate-duration (fig 6) and rate-number of kernels/m2 (fig 7) were proposed to test the grain weight and grain filling period for different environmental conditions and genotypes.