1. The administration of the caffein group, caffein, theobromin or theocin, to dogs in water balance results, not in diuresis but in a decrease in daily urine output, often to one-half the normal amount. The sodium chloride as well as water is decreased. Urea and total nitrogen are generally but not always lessened. If a single dose is given the diminished output is seen on the day of administration. If the administration is kept up through a number of days, the decrease persists through the period of administration.
2. The lessened output from the caffein group is not due to renal damage, since the kidney is able to eliminate added water, salt, urea and phenolsulphonephthalein in a normal manner.
3. During the period of lessened output the blood shows evidence of some concentration, in that the water content tends to fall and the hemoglobin to rise.
4. The explanation given for these effects is that there is a shift of water and salt from the circulating blood into the tissues. The action of the caffein group in bringing this about is to produce a dilatation and increased permeability of capillaries in the splanchnic organs and muscles. This action overshadows the kidney action in animals in water balance. The diuretic action appears when there is an excess of water in the body or when the tissues give off some of their stored water.