Because ascorbic acid (AA) and the thyroid hormones are known to influence eggshell formation, the effects of AA on body weight, total plasma thyroxine (T4), egg production, and eggshell quality of Single Comb White Leghorn (SCWL) hens concurrently and previously treated with thiouracil (TU) were determined. Hens were provided feed containing either 0 or 100 ppm AA from 47 to 67 wk of age and either 0 or .1% TU from 47 to 57 wk of age. A three-way split-plot analysis was employed to test for the effects of AA, TU, and time, and their interactions. Dietary TU increased body weight between 53 and 59 wk, and increased thyroid weight at 67 wk; however, TU only depressed plasma T4 level at 51 wk. Dietary TU depressed egg production from 50 to 56 wk and egg weight at 49, 51, and 57 wk. Dietary TU depressed eggshell weight per unit surface area at 49 wk. There was no significant effect due to AA and no AA by TU interaction for any of the parameters examined. It was concluded that .1% dietary TU from 47 to 57 wk of age did not alter eggshell quality and that dietary AA at the 100 ppm level did not influence the effects of .1% TU on body weight, egg production, or egg weight of SCWL hens.