Chloride and potassium activities inside the lumen and the transepithelial potentials of control and drug-treated turtle thyroid follicles were determined simultaneously by ion-selective and conventional 3 M-KCl micro-electrodes, respectively. Water and electrolyte contents of these thyroid tissues were determined chemically after each experiment. Cellular and luminal concentrations of Cl- and K+ in control and drug-treated thyroid glands were derived from the data obtained. Both Cl- and K+ equilibrium potentials across the follicular cell membranes calculated from their concentration gradients are not identical to their corresponding membrane potentials measured directly. Thus, transport of both ions occurs. Thyrotrophin, SITS and ouabain increased both cellular and luminal Cl- concentrations. Acetazolamide increased the cellular but did not alter the concentrations. Furosemide and perchlorate markedly increased the cellular but decreased the luminal Cl- concentrations. There is a discrepancy in the intracellular K+ concentration between values derived from electrometric data as measured by ion-selective micro-electrodes and those calculated from the chemical analyses. The electrometrically determined intracellular K+ concentration is smaller. Intracellular Cl- concentrations derived from measurements with the Cl- ion-selective micro-electrodes were in the same range as those calculated from chemical analyses. Thus, either method provides accurate values for intracellular Cl- concentrations.