Congestive heart failure was produced in nine dogs following surgical removal of the tricuspid valve. Six dogs developed ascites with or without peripheral edema, and four dogs remained ascites-free for 5, 6, and 11 weeks (one dog in this latter group did later develop ascites 14 weeks following operation; it was therefore studied twice and included in both groups). Systolic aortic pressure, sodium balance, right atrial pressure, glomerular filtration rate, sodium para-aminohippurate clearance, plasma aldosterone, norepinephrine, renin activity, and antidiuretic hormone levels were measured in each dog before surgery and during the experimental period of the steady state of edema in the dogs of group A, and at 5, 6, and 11 weeks in the four dogs of group B. Right atrial pressure increased considerably and glomerular filtration rate and sodium para-aminohippurate clearance decreased markedly in each dog. The most significant and important findings were (1) normal sodium balance in the dogs of group B without ascites, whereas four of the six dogs with ascites had a markedly positive sodium balance; (2) increased antidiuretic hormone activity levels in all dogs with congestive heart failure and ascites in contrast with the absence of any change in the dogs without ascites. The changes in plasma aldosterone, norepinephrine, and renin activity levels were not consistent. It is suggested that the antidiuretic hormone may play a role in the formation of ascites and edema in dogs with congestive heart failure following total removal of the tricuspid valve.