The parameters which are important in causing changes in the Tl and T2 proton magnetic relaxation times of dog bile were investigated. Three factors were found to be important in causing relaxation in bile: (i) total bile salt concentration; (ii) total protein concentration, and (iii) viscosity. The T1 and T2 values of hepatic and gallbladder biles were found to be independent of specific gravity, osmolarity and electrolyte concentrations. In vitro experiments were conducted with taurocholic acid, bovine serum albumin and porcine stomach mucin to examine the effects of intermolecular interactions on proton relaxation. Relative to each of the molecules alone, various combinations of the bile salt and proteins exhibit relaxation rates of 20 to 60% below theoretically expected values. This influence of in vitro molecular interactions on T1 and T2 is also likely to occur in hepatic and gallbladder biles in vivo. Thus, the effects of complex intermolecular interactions associated with the gallbladder microenvironment complicate but likely will not preclude direct assessment of physiologic data with magnetic resonance imaging.