Aims: The p38 mitogen-activated protein kinase (MAPK) has been shown to be important for fast MyHCIId/x promoter activity in C2C12 myotubes, which represent a fast fibre type in terms of myosin heavy chain (MyHC) expression. We now asked whether p38 MAPK is involved in the regulation of the slow MyHCI/b promoter. Methods: p38 MAPK activity was inhibited by 1 mM of the specific compound SB203580, and activated by overexpression of a constitutively active mutant of direct upstream MAPK kinase 6 (MKK6), MKK6EE. Intracellular localisation of endogenous nuclear factor of activated T-cells isoform c1 (NFATc1) was assessed by immunofluorescence analysis. The activity of a -2.4 kb MyHCI/b promoter luciferase construct was determined in transient transfection assays in C2C12 myotubes. Results: NFATc1, which has been shown to be important for MyHCI/b promoter activation, is nearly completely localized in the cytoplasm of untreated myotubes. Treatment with SB203580 led to partial nuclear translocation of NFATc1 and to a moderate increase of MyHCI/b promoter basal activity. Therefore, p38 MAPK is involved in keeping the basal level of MyHCI/b low. The region responsive to p38 MAPK action is located in the proximal promoter region. Addition of Ca²⁺-ionophore inhibited p38 MAPK activity, induced nuclear import of NFATc1, and a fast-to-slow transformation on the level of MyHC promoter activity. Activation of p38 MAPK by overexpression of MKK6EE decreased nuclear localisation of NFATc1 and impaired Ca²⁺-ionophore-induced MyHCI/b promoter activation. By use leptomycin B, a highly specific inhibitor of CRM1-dependent nuclear export, we could demonstrate that MKK6EE can counteract the Ca²⁺-ionophore-induced NFATc1 nuclear import by promoting nuclear export. Conclusion: The data indicate that p38 MAPK can negatively affect MyHCI/b promoter activity by promoting cytoplasmic localisation of NFATc1. These effects of p38 MAPK are moderate, indicating that p38 is not a major NFATc1 kinase.