Muscles subjected to different patterns of electrical activity from the CNS will change its properties, but the signalling pathways connecting activity and alterations in gene expression are still not well understood. Fast muscles have a higher level of the muscle specific transcription factor MyoD than slow muscles. MyoD can be inactivated by phosphorylation. When fast muscles of rats is subjected to a slow, but not a fast pattern of electrical stimulation, we found that MyoD is phosphorylated and thus inactivated. After chronic stimulation, both the mRNA and the total protein level of MyoD protein were decreased, while the level of phosphorylated, inactivated MyoD was increased. When wild type MyoD was over-expressed in normally active slow solei it was relatively inefficient in inducing "fast" properties, but removing the "slow" activity by denervation induced fast fibre types. A mutant form of MyoD, T115A, which is not sensitive to phosphorylation, was very efficient in activation of fast MyHC genes in vivo, irrespective of activity. These findings suggest that MyoD activity is regulated by activity-dependent phosphorylation in a pattern specific manner, and that active MyoD can induce expression of fast contractile proteins.