University of Wales, Bangor, School of Sport, Health and Exercise Sciences, LL57 2PX Bangor, United Kingdom Skeletal muscle cells adapt to physiological demands by changing their myosin composition, metabolic profile and diameter. The physiological stimuli and the signal transduction of the different adaptational responses are not known in great detail. We have used a primary skeletal muscle cell culture growing on microcarriers to study the role of passive mechanical forces and contractile activity on myosin heavy chain and on IGF-1 transcription, a mediator of skeletal muscle hypertrophy. Additionally we studied the significance of integrin as a possible sensor of passive mechanical forces. These passive forces were generated by continuous movement of the culture flasks on a rotating shaker, and contractile activity was elicited by electrostimulation of the myocytes. We find that passive mechanical forces induce IGF-1 transcription as well as transcription of fast myosin heavy chain IId, while slow MHCI is unaffected. Specific patterns of electrostimulation induce MHCI but not MHCIId, and this induction is suppressed during exposure to passive mechanical forces. Inhibition of integrin function by echistatin reduces the induction of IGF-1 transcription by