The ionic conductance mechanism underlying bursting pacemaker activity was investigated in cultured AtT-201D16-16 (D16) anterior pituitary cells. D16 cells secrete the ACTH/B-endorphin family of peptides in response to Ca influx. The influx of Ca was found to be coupled to rhythmic Ca spikes triggered by an underlying pacemaker oscillation. The basis for the oscillation appears to be the activation of two classes of Ca gated channels. One of these has a large single channel conductance (130 pS) and is K selective. The other has a smaller unitary conductance (25 pS) and is nonselective to monovalent cations. The interplay of these two channel currents appears to account for spontaneous activity, whereas the Ca influx from the Ca spikes serves to couple membrane excitation to hormone secretion.