A versatile Wheatstone bridge is described for alternating current impedance measurements at frequencies from 30 to 10,000,000 cycles. Variable electrolytic resistors have proven very satisfactory both for balancing the bridge and as secondary standards of resistance. A selector switch in each working arm permits any one of nine different combinations of impedances to be connected in the arm so that it is possible to make either direct or substitution measurements against either wire wound or electrolytic resistors. Substitution measurements are made with an electrolytic resistor over the frequency range in 20 minutes, with an impedance accuracy of 0.2 percent. The equipment was designed and has been used primarily for impedance measurements of biological materials.