A new model of stretch-induced growth is evaluated in four chicken wing muscles stretched to different extents by a spring-loaded tubular assembly. Muscles grew in length and cross section in proportion to the extent to which they were stretched. Longitudinal growth was essentially completed within 1 wk, while muscles grew in cross section through at least 5 wk of stretch. The muscles were neither denervated nor immobilized, and muscle activity as measured by EMG was not increased. Oxidative enzyme activities increased substantially with stretch in the patagialis (PAT), a twitch muscle, but were relatively unchanged in the slow-tonic anterior latissimus dorsi (ALD). Stretch altered mitochondrial enzyme proportions in the PAT, but had little effect in the ALD. Capillary density was unchanged with stretch in the PAT, but decreased in the ALD. Capillary density was unchanged with stretch in the PAT, but decreased in the ALD. Capillary-to-fiber ratio, however, increased in both muscles. We conclude that muscles grow and adapt enzymatically due to stretch, but that these responses are dissimilar in twitch and tonic muscles.