The aims of our experiments were (1) to study age-dependent changes in bovine plasma concentrations of insulin-like growth factor I (IGF-1), thyroid hormones (T3, T4), some metabolites (glucose, triglycerides, ketone bodies), calcium, phosphorus and sodium, and (2) to determine whether plasma concentrations of these substances are associated with individual and sex-dependent differences in the growth rates of calves. Eight male and 8 female calves were weighed at the beginning and at the end of the experiment and daily gain and feed consumption (dry matter, crude protein, PDIN and NEL per kg daily gain) were determined. Plasma concentrations of IGF-1, T3, T4, glucose, triglycerides, ketone bodies, calcium, phosphorus and sodium were measured in calves at 2, 4, 6, 8, 10 and 12 weeks of age. There were significant age-dependent increases in the plasma concentrations of IGF-1 and glucose. Males had a significantly higher daily gain and lower feed consumption than females, but no sex-dependent differences in plasma constituents were detected. In both sexes, daily gain was positively correlated with plasma IGF-1 (coefficients of correlation r=0.630 in females and r=0.614 in males) and glucose (r=0.496 and 0.427, respectively) concentrations, but not with other substances. Our observations show that there are age-dependent changes in IGF-1 and glucose concentrations (but not in those of other substances), and that IGF-1 and glucose, in contrast with other substances, may be indicators or regulators of individual growth rates.