The mouse AtT-20/D16-16 anterior pituitary tumour cell line was employed as a model system for the study of the mechanisms mediating calcium ion-stimulated adrenocorticotropin (ACTH) secretion. The present study indicates that calcium ion-stimulated ACTH secretion from AtT-20 cells is mediated by a GTP-binding protein which is present in a variety of other cell types and has been dubbed Ge (for reviews see Gomperts, 1990; Lindau & Gomperts, 1991). In AtT-20 cells the nature of Ge remains elusive with the selective heterotrimeric GTP-binding protein activator AIF(3-5) proving not to be a useful pharmacological tool under the conditions employed in the present study. Ge present in this cell line does however display characteristics consistent with it being a heterotrimeric GTP-binding protein. The results of this study would also suggest that in AtT-20 cells Ge is insensitive to both pertussis toxin and cholera toxin. Both cyclic AMP-dependent protein kinase (PKA) (Guild, 1991) and protein kinase C (PKC) (Guild & Reisine, 1987; Reisine, 1989) have been implicated in the regulation of calcium ion-stimulated ACTH secretion from AtT-20 cells. Results from the present study suggest that calcium ion/Ge-stimulated ACTH secretion from AtT-20 cells is not mediated by PKA, PKC or any other kinase but is in fact mediated by a phosphatase. PKC appears to provide a direct stimulus to secretion, which is independent of calcium ion/Ge-stimulated secretion, in contrast to PKA which is unable to stimulate secretion by itself but seems to play a modulatory role with regard to both calcium ion/Ge- and PKC-stimulated secretion.