l-Thyroxine is converted to 3,5,3'-l-triiodothyronine (T(3)) as well as to 3,3',5'-l-triiodothyronine (reverse T(3)). One product of further deiodination is 3,3'-diiodothyronine (3,3'T(2)). The serum levels of reverse T(3) and 3,3'T(2) change considerably in various physiological and disease states. We previously found that reverse T(3) and 3,3'T(2) bind to the solubilized hepatic nuclear "receptors" for thyroid hormones. This led us to study binding and actions of these metabolites in cultured rat pituitary cells in which glucose consumption and growth hormone production are regulated by T(3) and l-thyroxine. Reverse T(3) and 3,3'T(2) stimulated growth hormone production and glucose consumption and inhibited nuclear binding of radioactive T(3). Either metabolite produced maximal effects that equaled those of T(3), and neither inhibited the T(3) response. Further, additive effects were observed when reverse T(3) was combined with submaximal concentrations of T(3). In serum-free and serum-containing media, concentrations of 3,3'T(2) 50- to 70- and 10- to 100-fold greater, respectively, than those of T(3) were required for equivalent stimulations and for inhibition of nuclear binding by T(3). The relative activity differences under the two conditions can be attributed to weaker serum protein binding of 3,3'T(2) than T(3). With cells in serum-free media, reverse T(3) was a less avid competitor than 3,3'T(2) for T(3) binding by the nuclear receptors, and was less potent than 3,3'T(2) (0.001 the potency of T(3)) in inducing growth hormone production or glucose oxidation. In incubations with serum-containing media, reverse T(3) was an ineffective competitor for T(3) binding, and had only 0.1 the inducing potency of 3,3'T(2) (0.001 the potency of T(3)). The weaker activity of reverse T(3) relative to 3,3'T(2) in serum-containing media could be explained by stronger serum binding of reverse T(3) than 3,3'T(2). In addition, after long-term incubation of cells with radioactive reverse T(3), much of the cell-associated radioactivity was recovered as 3,3'T(2). These studies suggest that reverse T(3) and 3,3'T(2) can stimulate thyroid hormone-regulated functions as weak agonists by acting via the same receptors that mediate T(3) actions. Moreover, some of the effects of reverse T(3) may be due to 3,3'T(2) produced by deiodination of reverse T(3).