The interest of the authors in fluorine dates from the discovery by Ostrem, Nelson, Greenwood, and Wilhelm of fluorine in the drinking waters of communities in the State of Iowa, with the resulting production of mottled enamel of the teeth. Smith, Lantz and Smith have produced considerable evidence that excessive fluorine in water will cause mottled enamel in man. The question arises as to whether fluorine produces other harmful effects in the body. The following work relates to the effect of fluorine on respiration, blood pressure, hemoglobin, blood calcium, blood phosphorus, and coagulation of blood. Sollmann refers to the work of Tappeiner and states that in mammals NaF causes salivation, gastroenteritis, dyspnea, muscular weakness and tremors, epileptic convulsions, fall of blood pressure, and stoppage of respiration and heart. Schulz observed similar results. Gerschmann reports that injection of NaF reduces blood calcium. Stuber and Lang observed hemophilic patients with high amounts of F in the blood, and they accept the idea that high F content of blood accompanies a long clotting period. Schwyzer observed a high degree of coagulability of the blood of rabbits, dogs, and pigeons which received F. Hauck, Steenbock, Lowe, and Halpin reported analyses of the calcium and phosphorus of the blood of chickens receiving fluorides, but they admit that their data are insufficient to draw any definite conclusions. Phillips states, “Very little work is reported dealing with the influence of fluorine on blood composition.” Seven dogs, 5 females and 2 males, were employed in studies on respiration and blood pressure. They weighed from 6 to 26.8 kg. The remainder of the experiments were performed on puppies—13 in number, 5 males and 8 females. Nembutal in distilled water was used as the anesthetic. It was administered intraperitoneally at the rate of 30 mg. per kilo body weight. NaF was injected into the femoral vein. Blood pressure records were taken from the central (heart) end of a ligated carotid artery with a mercury manometer writing on a long paper kymograph. Eight per cent citrate solution was used to prevent clots in the cannulae and tubes.