Retinoid X receptor-(RXR ) is a transcription factor that mediates retinoid signalling and is expressed in rat heart during adult life. However, its expression in embryonic and neonatal heart has not been investigated and it is not known whether ventricular cardiomyocytes express RXR or whether all-trans-retinoic acid (tRA) and thyroid hormone (T 3 ) could influence RXR transcript levels in these cells. First, in situ hybridization experiments were used to test for any spatio-temporal correlation between RXR gene expression and the previously shown requirement for retinoid signalling in embryonic ventricular cardiomyocytes. It was shown that RXR transcripts are not detectable in embryonic heart at all developmental stages examined under conditions where they are detectable in other embryonic tissues. Second, Northern blotting was used to examine whether there is a difference in RXR transcript levels between neonatal and adult heart. We show that levels of two RXR transcripts are developmentally regulated during the postnatal period because they differ between neonatal and adult hearts. Third, it was demonstrated that primary cultures of neonatal rat ventricular cardiomyocytes express RXR transcripts, making them a novel in vitro system for the study of RXR gene regulation in heart-derived cells. Finally, this system was used to examine whether tRA and T 3 can influence levels of RXR transcripts because they have been shown to have antagonistic effects in this system and to influence RXR RNA levels in other systems. It was shown by Northern blot experiments, that in this system, RXR transcript levels are differentially influenced by these two hormones. The significance of these findings in relation to previously published work is discussed.