Triiodothyronine (T3) stimulates a marked increase (\40-fold) in transcription of the malic enzyme gene in chick embryohepatocytes (CEH), but has no effect on malic enzyme transcription in chick embryo fibroblasts (CEF) that express nuclear T3 receptors (TR) at levels which are similar to those of CEH. Heterodimerization of the TR with other nuclear proteins is a potential mechanism for the regulation of T3 action. For example, heterodimers of retinoid X receptors (RXR) and TR bind to T3 response elements (T3RE) with higher affinity and modulate transcription more effectively than TR homodimers. In the present report, we investigated the role of RXR in mediating differences in T3 responsiveness of the malic enzyme gene between CEH and CEF. Data from gel mobility shift analyses demonstrated that endogenous TRs from CEH and CEF bind to the major T3RE of the malic enzyme gene primarily as heterodimers with RXRa or a protein highly related to RXRa. The total binding activity of RXRa: TR complexes in CEF was decreased relative to that observed in CEH. Cell-type dependent differences in RXRa: TR complex formation were greater in cells incubated in the presence of T3 because T3 treatment increased RXRa: TR binding activity in CEH but had no effect on RXRa: TR binding activity in CEF. Decreased RXRa: TR complex formation in CEF relative to CEH was associated with a reduction in the abundance of RXRa protein and RXRa mRNA in the former cell-type. Expression of exogenous RXRa in CEF increased the T3 responsiveness of the malic enzyme promoter by about 4-fold. In contrast, expression of exogenous RXRa in CEH had no effect on the regulation of malic enzyme transcription by T3. These observations support the hypothesis that alterations in RXRa expression contribute to cell-type dependent differences in T3 responsiveness of the malic enzyme gene.