In isolated ventricular cells from the adult guinea pig heart the slow Ca current was recorded during beta-adrenergic stimulation and during cell dialysis with a protein-specific phosphatase-1 (PPase-1). The increase in the amplitude of ICa during bath application of isoprenaline (5 X 10(-8) M) could be completely reversed by dialysing the cell with 2 microM PPase-1. Lower enzyme concentrations produced smaller effects. The control amplitude of ICa was only little affected by dialysis with PPase-1. The result suggests that PPase-1 is a likely candidate for the downregulation of Ca channels, that Ca channels can open in the dephosphorylated state.