A hand held breath analyzer is energized when a mouthpiece tube is put into position. An ejector mechanism expels the mouthpiece, which de-energizes the system. The insertion of the mouthpiece not only cocks the ejector mechanism but closes a cover on an exhaust manifold of a breath sample passage. The breath sample passage includes a sample chamber with a port to receive a sampling tube fitting, and an auxiliary port. The exhaust manifold is long relative to the length of the sampling chamber and terminates in an orifice by means of which ambient air is drawn across a thermistor. The sampling tube communicates with a fuel cell chamber, into which breath is drawn by a diaphragm pump arranged, in its normal, uncocked condition, to inhibit the entrance of contaminants into the chamber. The diaphragm pump is actuated in response to operation of the thermistor, through a relay-tripped toggle linkage. A flag, connected to one of the toggle members, blocks the path of light from a photocell to a photoreceptor, insuring that the diaphragm pump is properly cocked for use before a breath sample is attempted to be taken.