Prolactin receptor and oestrogen receptor are co-ordinately expressed in human breast cancer cell lines and in human breast tumour biopsies, leading to the suggestion that the expression of these receptors may be coupled. To examine this hypothesis, T-47D and MCF-7 human breast cancer cells were treated with sodium butyrate, a known modulator of oestrogen receptor levels, and the changes in oestrogen and prolactin receptor mRNA and binding activity were measured. In both cell lines treatment with 0.3-10 mM sodium butyrate resulted in a parallel decrease in prolactin and oestrogen receptor mRNA levels and binding activity. In T-47D cells, where the effect was transient, mRNA levels of both receptors recovered in parallel. These data indicated that oestrogen receptor and prolactin receptor gene expression is modulated in parallel by sodium butyrate and supported the hypothesis that the expression of these two receptors is coupled.