A method is described to activate the systolic rise of [Ca2+]i in only one region of a single, isolated cell. This is achieved by applying the calcium chelator BAPTA to the rest of the cell from a pipette. Under control conditions electrical stimulation produced a Ca transient which was uniform throughout the cell. If a BAPTA containing solution was applied to one region of the cell for 100-500 ms before stimulation then there was no systolic Ca transient in that region of the cell. In the rest of the cell, however, the Ca transient was identical to that in control conditions. If BAPTA application was discontinued the Ca transient was normal throughout the cell on the next stimulation. In the presence of ouabain the locally activated systolic Ca transient propagated through the cell. Propagation was associated with an increase of systolic but not diastolic [Ca2+]i. These results show that the systolic Ca transient propagates if the cell Ca content is elevated. We suggest that the fact that Ca-overload produces spontaneous Ca release may be due to the fact that it allows spontaneous Ca release (which may always be occurring) to propagate.