Conditions of survival and iodide trapping capabilities of sheep and dog thyroid slices were studied during incubations in vitro. Preincubated dog thyroid slices offered the best survival and function up to 6 hours of incubation. The present system with continuous recording of experimental data by a multichannel analyzer, allows compartmental studies by means of computer analysis. Autoradiographies from dog thyroid slices incubated in vitro with radioiodine provided evidence of heterogeneity of function by thyroid follicles. TCA: trichloroacetic acid. 1 Work realized under Contract of the Ministere de la Politique Scientifique within the framework of Contract Euratom - Universities of Brussels and Pisa BIAC (n° 026-63-4).