The effect of 1.7-2.2 mg/day oral phenobarbital over short (1 MO) and long term (6-24 MO) treatment on primary bile acid (BA) secretion, composition, synthesis, pool size, and enterohepatic cycling rates as well as phospholipid (PL) and cholesterol (C) secretion rates and biliary composition was determined in 12 asymptomatic cholesterol gallstone subjects while 5 normals had only short term studies. Phenobarbital enhanced BA and C secretion (BA-636 +/- 166 to 2110 +/- 382 mg/hr, p less than 0.001 and C-42 +/- 5 to 224 +/- 48 mg/hr, p less than 0.001) and BA cycling rate in all subjects studied during stimulated enterohepatic circulation but, during fasting, it only enhanced BA secretion (451 +/- 129 vs. 759 +/- 159 mg/hr, p less than 0.05) in gallstone subjects. Cholic acid (CA) production rate (171 +/- 28 to 395 +/- 9 mg/hr, p less than 0.05) and pool size (727 +/- 80 to 1209 +/- 132 mg/hr, p less than 0.05) were increased during long term treatment of gallstone subjects, while the proportion of CA in bile and deoxycholic aicd (DCA) in feces increased. Treatment decreased biliary cholesterol from supersaturated to saturated levels (9.5 +/- 0.6 vs. 6.1 +/- 0.9 moles %, p less than 0.02) in all fasting gallstone subjects and decreased cholesterol crystal loads during long term treatment; but, while prohibiting gallstone growth, it did not affect stone dissolution over 24 months' treatment. Phenobarbital also failed to affect biliary lipid composition or bile acid pool size in short term treatment of normals. Thus, phenobarbital affected hepatic metabolism of CA by enhancing production rate, secretion, and pool size; and intestinal metabolism of both CA and chenodeoxycholic (CDC) acids by increasing their cycling rates. Phenobarbital may have failed to produce stone dissolution by enhancing CA production and pool size more than that of CDC.