Studies were carried out to characterize changes in MAP kinase activation during contraction of collagen matrices by fibroblasts under isometric tension. We found that both ERK and p38 MAP kinases were activated during contraction, as determined by immunoblotting and in vitro kinase assays. ERK activation was biphasic, with peaks at 10 min and 2 h; whereas p38 activation was monophasic, with a single peak at 10 min. Activation of ERK, but not p38, appeared to depend at least in part on the Gi class of heterotrimeric G proteins. The results show that ERK and p38 cooperate in contraction-stimulated activation of c-fos transcription.