Summary.The biological action of l‐triiodothyronine, azo‐arsenophenyl‐triiodothyronine and azo‐sulfophenyl‐triiodothyronine was determined on the basis of their positively chronotropic effect on the isolated spontaneously beating rat auricle after a three days treatment of the animals with the substances in question. It is evident that triiodothyronine has an accelerating effect on the rate of beat similar to that of thyroxine. An effect of the same kind but weaker, could also be demonstrated with the azo