The p21-activated protein kinases (PAKs) may participate in signalling from Cdc42/Rac1 to the stress-regulated MAPKs (SAPKs/JNKs and p38-/HOG-1-related-MAPKs). We characterized the expression and regulation of αPAK in cultured ventricular myocytes. αPAK was specifically immunoprecipitated from myocyte extracts. High basal αPAK activity was detected in unstimulated myocytes. Its activity was increased rapidly (<30 s) by hyperosmotic shock in the presence of okadaic acid, and was maximal by 3 min (187±7% relative to unstimulated cells). Endothelin-1 and interleukin-1β, which also activate SAPKs/JNKs, did not increase αPAK activity and presumably act through different PAK isoforms or other mechanisms. © 1997 Federation of European Biochemical Societies.