The mean values for six adult puffins were hematocrit, 0.511 ± 0.007; plasma sodium, 160.7 ± 2.3, potassium, 3.1 ± 0.1, and chloride, 133.4 ± 1.9 meq/l.The cloacal fluid of one young laboratory-reared puffin was always hypotonic to the plasma in Na and Cl and never contained more than one-half the ingested Na, K, or Cl; contained less than 40% of the ingested water; and solid material was less than 10% of the total as determined by dry weight.In one salt load experiment (4.0 ml 1.0 N NaCl/3 92-gram adult) the salt glands proved very efficient, producing a secretion of 975 meq/l (maximum) Cl and eliminating 75% of the load in 20 min. The salt glands and kidneys are large, 0.1% and 1.8% of the body weight respectively.