Serum concentrations of triiodothyronine (T3), thyroxine (T4), and TSH were examined in seven men and seven women of normal weight during a 60-hr fast. Similar studies were conducted in two women who received daily for 1 mo before and during a similar fast, 0.4 mg and 0.5 mg of 1-thyroxine. The serum concentrations of T3 decreased in each of the untreated normal subjects (sign test of significance, P less than 0.001). The mean control concentration of T3 in women was 152 +/- 9 ng/100 ml (X +/- SEM); after 24 hr of fasting, 131 +/- 31 ng/100 ml; and at the termination of the fast, 90 +/- 15 ng/100 ml. The latter value differed from the control value with a p value of less than 0.01. Similar changes of T3 concentration occurred in men (mean basal T = 160 +/- 11 ng/100 ml; mean at termination of fast = 87 +/- 16 ng/100 ml). The range of decrease for T3 in all subjects varied from 24% to 55%. The mean T4 concentration at the beginning of the fast was 6.9 +/- 0.9, and at the termination of the fast, 7.5 +/- 0.6 (p = NS). TSH concentrations remained unchanged (Control, 3.8 +/- 0.45 muU/ml; at 60 hr, 4.0 +/- 0.26 muU/ml, p = NS). Studies in two women who received, before and during a fast, T4, indicate that a decreased peripheral conversion of T4 to T3 is the most likely mechanism responsible for this change.