The mammalian Cdc42 protein regulates various kinds of cellular responses, including formation of filopodia, polarization of T cells, and cell cycle progression. A non-receptor tyrosine kinase ACK, which specifically binds to the GTP-bound form of Cdc42, was isolated as a putative target of Cdc42. Here we show the induction of tyrosine phosphorylation of ACK in response to temperature shift-down to 25 degrees C, and hypertonic shock, as well as stimulation with epidermal growth factor (EGF) in human embryonic kidney (HEK) 293 cells. The increased tyrosine phosphorylation level upon temperature shift-down was sustained for at least 60 min, whereas reversion of the temperature to 37 degrees C caused rapid tyrosine dephosphorylation to the initial level. The responses to EGF and the high osmolarity were transient. Furthermore, we observed association of ACK with an adaptor protein Grb2, which may suggest the involvement of Grb2 in EGF receptor-mediated tyrosine phosphorylation of ACK.