The reciprocity pressure response formula is derived from the equations of motion of the electroacoustic systems involved in reciprocity measurements in which only two of the three transducers are coupled at a time to the chamber. The formula includes the acoustical impedances of the three transducers. When it is assumed that the acoustical impedances of the transducers are large relative to the chamber impedance an approximate formula is obtained which includes the acoustical impedance of only one of the transducers, the one whose response is given by the formula.