SUMMARY: Adult rats in groups of ten were maintained variously on full, half and quarter food intake. Some were given plain drinking water ad libitum, others were given a goitrogenic challenge of 0·1% aminotriazole (ATA) in the drinking water. Food and fluid consumption were measured daily. The animals were killed after 3 weeks for measurement of thyroid and body weight. The thyroids were histologically examined and epithelial cell height was measured. Another series of rats on normal food intake were given ATA solutions of various concentrations to drink and the efficacy of the concentrations to depress a 16 h thyroid uptake of 131I was measured. The results showed a reduction in average thyroid cell height parallel to the degree of starvation in the non-ATA treated groups and a reduction in goitrogenic response to equivalent challenge with ATA of about 15% in the half-fed animals and 20–25% in the quarter-fed animals. It is suggested that, in adapting to caloric restriction, higher centres depress the thyrotrophic hormone releasing factor response of the hypothalamus to a fall in circulatory thyroid hormone concentration. The degree of reduction in goitrogenic response might be regarded as a quantitative index of this depression.