These studies correlate the effects of (sodium) butyrate on intranuclear thyroid hormone receptor levels, with influences on both endogenous and transfected rat growth hormone (rGH) gene expression and regulation by L-triiodothyronine (T3). In rat anterior pituitary tumour (GH3) cells, 5.0 mM butyrate elicits a biphasic reduction in the number of nuclear T3 receptors. About 75% are depleted rapidly (t1/2 = 7 h), and the remaining receptors are depleted more slowly (t1/2 = 59 h). GH3 cells were treated with increasing concentrations of butyrate (0-5.0 mM), plus or minus 10 nM T3 for 48 h. Total cytoplasmic RNA, cellular protein and medium were analysed for rGH levels with radiolabelled rGH cDNA or antibodies. A greater than 50-fold increase in rGH mRNA level was seen after T3 treatment in the absence or presence of 0.1 mM butyrate. However, 1.0 and 5.0 mM butyrate decreased the stimulation of rGH mRNA levels by T3 to 10- and less than 2-fold, respectively. Control mRNA levels were decreased slightly by increasing butyrate concentrations; rGH mRNA level was 2- to 3-fold higher in the absence of 5 mM butyrate. The pattern of butyrate/T3 response displayed by both cellular and secreted rGH was similar to that seen with mRNA levels. Thus, the predominant effect of butyrate on T3-mediated regulation of growth hormone gene expression is at the level of transcription or mRNA accumulation. A hybrid gene containing 5'-flanking DNA from the rGH gene fused to the bacterial gene coding for chloramphenicol acetyl transferase (CAT), was used to transfect rat pituitary tumour cells with or without butyrate and T3 treatments.(ABSTRACT TRUNCATED AT 250 WORDS)