Mature, single muscle fibres were dissected from Xenopus leavis m. iliofibularis and attached to a force transducer in a 20 ordm;C culture chamber. Fibres were cultured at a mean sarcomere length of 2.3 mm for 10 to 22 days at the following conditions:(1) without growth factors, (2) with bovine insulin, (3) with recombinant insulin-like growth factor 1 (IGF-1), (4) with stabilized mechano growth factor E-domain (MGF E-domain), (5) with insulin and MGF E domain or (6) with IGF-1 and MGF E-domain. In muscle fibres cultured in the presence of either insulin, or IGF-1 or with a combination of insulin or IGF-1 and MGF E-domain, tetanic force increased up to 16.5±1.6% (mean±SEM) after 16.6±0.6 days of culture. This was accompanied by an increase of the cross-sectional area of 33.1±2.2% (P<0.0001). For all conditions, the number of sarcomeres in series remained unchanged during culture. After culture, for fibres exposed to insulin and IGF-1, the number of myonuclei per nL of cytoplasm was 36% lower compared to that of fibres cultured in the presence of insulin or IGF-1 with MGF E-domain (P<0.001). We conclude that insulin and IGF-1 induce hypertrophy of muscle fibres without an increase of the number of myonuclei, whereas MGF E-domain stimulates an increase of myonuclei without inducing hypertrophy.