Muscle growth of animals is to some extent governed by systemic hormonal effects. In the present work we have related serum IGF-I and IGF-binding proteins (IGFBP) as well as overall mitogenic activity of serum to characteristics of the muscle that are associated with the growth processes. Serum was obtained from animals coming from two experiments, i.e. 20 prepubertal gilts slaughtered at different ages (5 to 181 days of age), and 12 male and 12 female littermates slaughtered at 100 kg live weight. From the prepubertal gilts, muscle samples were obtained at slaughter. Serum or plasma was assayed for IGF-I and IGF-binding proteins (IGFBP), and the mitogenic activity of serum was evaluated using a cloned muscle cell line (L6). Muscle samples were assayed for nuclei and type I IGF-receptors. In agreement with a higher growth rate of male pigs, the mitogenic activity of serum from male pigs was higher (27%, P=0.03) than that of female littermates. Serum from male pigs contained more IGF-I and less IGFBP-2 than did serum from female pigs. In prepubertal gilts, plasma IGF-I and IGFBP-3 levels increased with age while the other IGFBPs were unrelated to age. The mitogenic activity of serum tended to increase with age (r=0.43, P=0.06) and was positively correlated with plasma IGF-I (P=0.02), tended to be negatively correlated with IGFBP-2 (P=0.09)and positively correlated with IGFBP-3 (P=0.11). The number of nuclei per muscle fibre increased linearly with age in M. longissimus dorsi (r=0.86, P<0.001). In M. vastus intermedius, the number of nuclei per muscle fibre increased linearly from birth to d 100 of age (P=0.002) after which no further increase occurred (quadratic effect, P=0.05). The number of type I IGF-I receptors decreased slightly with age in both white and red muscles. The present data indicate that members of the IGF family are related to the overall mitogenic activity of serum. The incorporation of nuclei into muscle fibres seems to level off with age despite increasing mitogenic activity of serum with age. This may in part be due to a decrease in type I IGF-receptor number in the muscles.