To clarify the role of insulin, IGF-I and TSH in thyroid cell regulation, their effects on amino acid transport were studied separately. These effects were noted and compared using both the Wistar rat thyroid cell line and human thyroid cell cultures. Insulin, IGF-I and TSH were able independently to induce the radiolabelled alpha-aminoisobutyric acid transport within the rat thyroid cells: TSH stimulated the amino acid transport in rat thyroid cells in a dose-dependent way from 1 pmol/l to 10 nmol/l. Similarly, insulin increased amino acid transport significantly from 0.17 nmol/l up to 0.17 mumol/l and IGF-I from 0.13 pmol/l up to 0.13 mumol/l. The combined effects of insulin and TSH on amino acid transport were equal to the theoretical sum of the activities, whereas those of IGF-I and TSH were greater than the theoretical one. When human thyroid cell cultures were used, a significant increase of labelled amino acid transport was induced by TSH, i.e. from 0.1 pmol/l to 10 pmol/l; IGF-I stimulated amino acid transport in a range from 0.13 pmol/l to 13 pmol/l, under the same conditions. Conversely, only large doses of insulin, i.e. 1.7 nmol/l, were able weakly to stimulate amino acid transport. When submaximal TSH and IGF-I doses were co-incubated in human thyroid cells, an additive effect on amino acid transport was observed.