An apparatus and method for measuring the blood glucose level of a diabetic using insulin is noninvasive. The apparatus includes two plethysmographs that sense the blood in two fingers simultaneously. The electrodes in the plethysmographs are immersed in conductive liquids. One liquid has electric conductivity equal to blood. The other liquid is more conductive than blood. Signals from the fingers are compared to signals obtained earlier when glucose levels were known from conventional measurements. An alternative embodiment employs a single plethysmograph with one set of electrodes in a surrounding chamber and a second set of electrodes against the skin surface.