Abstract: The testing apparatus is for measurement of ions in body fluids. The apparatus has a plurality of sample cups for containing fluids to be tested, a movable door for controlling access to the sample cups, and a plurality of measuring electrodes for immersion into the fluid to be tested. There is a hand operated device for moving the electrodes and the cups with respect to each other to alternately put the electrodes in a first position permitting access to the cups and in a second position in which the electrodes are immersed in the fluids to be tested contained in the cups. Means for mounting the cups in the testing apparatus are engaged with means for oscillating the cups independently of the electrodes. The oscillation fully exposes the electrodes to a representative sampling of the fluids within the cups and thus enhances the accuracy of the test being performed.