A tubular PVC membrane electrode for calcium without inner reference solution and a device for location of the reference electrode are described. In the flow-injection system, calcium is determined potentiometrically and then magnesium is determined by atomic absorption spectrometry. The electrode provides linear response to calcium in the range 5 × 10-5/2-10-1 M. On-line dilution of the sample allows magnesium determination in the range 0/2-10 mg l-. Flow rates between 3 and 6 ml min-1 are possible. The sampling frequency is 60/2-90 h-1. © 1986.