SUMMARY Six iodo-l-thyronines, viz. 3:3′:5′-triiodo-, 3:5-diiodo-, 3:3′-diiodo-, 3′:5′-diiodo-, 3-monoiodo-, and 3′-monoiodo-l-thyronine, have been compared with 3:5:3′-triiodo-l-thyronine, in mice by an anoxia method and in rats for antigoitrogenic activities and effects on rate of oxygen consumption. The only compound with any significant activity was 3:5-diiodo-l-thyronine. In the goitre prevention assay it had 3% (on a weight basis) of the activity of 3:5:3′-triiodo-l-thyronine.