Polyethylene tubing (i.d. 0.023 in.) was found to be of uniform bore. Therefore, it was possible to cannulate the gall bladder of anesthetized, common-duct-ligated mice and measure the rate of bile flow by marking off 10-min collection periods with thin black plastic tape. Rate of flow for each mouse (cm of tubing/10 min) was calculated from at least three consecutive 10-min flow periods during which the rate was maximal and linear. The rate for male chow-fed controls was 6.9 ± se 0.14. Mice fed a 1% cholesterol, 0.5% cholic acid (C-CA) gallstone-inducing diet for 2 days, 5 days, and 2 months showed rates of 10.6 ± 1.1, 15.5 ± 1.48, and 34.8 ± 3.8. The rate for females on chow and after 2 months on the C-CA diet was significantly higher than that for males. Treatment of females with testosterone and of males with estrone caused a reversal of the sex difference. Cholesterol concentration of hepatic bile was no higher after 2 months on the C-CA diet than after 2 days, but total cholesterol secreted per unit time (concentration x flow rate) was significantly elevated at 2 months.