In order to study the variability and heritability of directly measured traits (growth and body development traits) in performance test of Simmental bulls the data on 371 bulls born and tested over the period of 13 years were used in the analysis. The data were analyzed in order to estimate year and month of calving, herd of origin and group in test effect as well as error components. The components of variance were obtained using restricted maximum likelihood (REML) methodology applied to sire model. The year and month of birth had different effect on the variability of the growth traits, while the herd of origin and the test group manifested a consistent, highly significant effect on those growth traits which they could have an effect on. On the other hand, all the body development traits were under a constant and highly significant effect manifested by the year of calving, while the month of calving manifested its effect, during test, at different levels of statistical significance. A decreased effect of herd of origin on almost all body dimensions from the start until the end of performance test was universally observed. Heritability estimates for pre-test ADG, in-test and lifetime ADG were 0.27, 0.39 and 0.29, respectively. Heritability estimates for body weights were 0.23, 0.25, and 0.30 for birth weight, test-on weight, and test-off weight, respectively. Heritability estimates for test-off height at withers, circumference of chest, depth of chest and body length were 0.43, 0.30, 0.33 and 0.29.