The characteristics and regulation of monoamine oxidase (MAO) were studied in rat thyroid tissue. A measured Michaelis constant (Km) value of 102 ìmol/l was similar to the Km values found in other tissues. Maximal velocity (Vmax) was 1·028 nmol/mg protein per min. It is known that MAO is present as two isoenzymes, A and B, which are sensitive to clorgyline and deprenyl respectively. The in-vitro effect of graded concentrations of these selective MAO inhibitors was used to estimate the relative proportion of A and B isoenzymes. Clorgyline strongly decreased thyroidMAO activity at concentrations as low as 1 pmol/l while the effect of deprenyl was observed only at concentrations higher than 10 ìmol/l. These results indicated that MAO-A is the main form of the enzyme in the rat thyroid. In-vivo administration of L-thyroxine (5·6?224 nmol/kg) significantly reduced thyroid MAO activity at doses equal to or greater than those which have been reported to inhibit iodine output from the thyroid. Increased TSH levels, induced either by exogenous TSH or methimazole administration, resulted in a significant increase in thyroid MAO activity. Theophylline, a phosphodiesterase inhibitor and dibutyryl cyclic AMP were also able to stimulate MAO activity when administered in vivo. Iodide organification (protein-bound 131I) in vivo as well as the relative proportion of the different thyroid iodocompounds were not affected in animals with reduced or increased thyroid MAO activity induced by clorgyline or theophylline respectively. It was concluded that rat thyroid MAO activity is under the influence of TSH. The adenylcyclase?cyclic AMP system participates in the regulation of enzyme activity as one of the intracellular mediators. An important effect of MAO on thyroid hormone biosynthesis is unlikely since changes in MAO activity did not affect the iodine organification process.