The deacetylase inhibitor trichostatin A (TSA) has long been used to study the relationship between gene transcrip-tion and the acetylation status of chromatin. We have usedXenopus laevisoocytes to study the effects of TSA on glucocorticoid receptor (GR)-dependent transcription and we have related these effects to changes in the chromatin structure of a reporter mouse mammary tumor virus (MMTV) promoter. We show that TSA induces a low level of constitutive transcription.