Histometric measurements were made on individual nodules and the internodular thyroid on glands from 15 patients without evidence of endocrine thyroid disease: some of the thyroids had multiple nodules. Measurements of the surviving internodular thyroid tissue showed that the various components were each within the normal range for thyroids from patients with no evidence of thyroid disease, but there were considerable deviations within a few glands in the colloid:epithelium ratio that were associated with histological appearances of dilated or collapsed acini. The interpretation of these appearances presented difficulties, but it was concluded that the balance of evidence favoured the inference that the internodular part of the gland was under physiological growth control at the time of study. The nodules varied in size: most contained proportionately less epithelium and more colloid than the internodular thyroid. It was concluded that the nodules were unlikely to be secreting autonomously in these patients.