Summary The structure of the hepatic endoplasmic reticulum membrane following triiodothyronine administration to thyroidectomized rats has been probed employing the technique of spin labelling. Hormone-sensitive changes residing in the membrane proteins were observed. No simultaneous change was evidenced in the lipid region of the membrane. It is concluded that one the events associated with the mechanism of action of triiodothyronine in target organs involves either the de novo synthesis or a conformational change of membrane proteins.