Activin A-, AB- and B-responding cells were characterized by their responsiveness in cytosolic free calcium ([Ca2+])i) to four hypothalamic releasing hormones, CRH, GHRH, TRH and GnRH. First, rat pituitary cells responding to activin A, AB and B in [Ca2+]i were determined in a mixed population of pituitary cells. The populations of the activin A-, AB-, and B-responding cells were 13.9%, 9.3% and 13.2%, respectively. Overlapping of response among each population of activin- responding cells was present in some of the cells. The cells responding to activin A, AB, and B were then characterized by their responses to CRH, GHRH, TRH and GnRH. Most of the cells responding to activin A, AB, and B also responded to GHRH or TRH. These results reveal that there are distinct differences among each population of activin A-, AB- and B-responding cells and that there is still functional overlapping of responsiveness among these populations. The characterization of activin-responding cells suggests involvement of somatotropes and lactotropes in activin-induced biological events in the pituitary.