Bulls of dual-purpose cattle at performance-test stations were weighed at intervals of 30 days from birth to 420 days. Live weight was evaluated by orthogonal Legendre Polynomial (LP) and by Linear Spline function (SP). The fixed effects of test-day-year-station of weighing (TDS) and LP within station-year explain together 97% of variability. Variability of all components increases with age. The heritability is 0.28 on average and increases with the age of the animal. Correlations for body weights between different ages of the animal are high.