To determine if changes in ADP-ribosylation of the chromosomal proteins can influence the expression of specific genes, the effects of compounds that influence this modification were investigated on the expression of the growth hormone (GH) and prolactin (Prl) genes in cultured rat pituitary (GH3) cells. The drugs tested, nicotinamide, N'-methylnicotinamide, 5-methylnicotinamide, 3-acetylpyridine, and 3-aminobenzamide, decrease ADP-ribosylation either by inhibiting (ADP-ribose)n synthetase and/or by decreaseing cellular levels of NAD+, the substrate for the enzyme. These drugs increased the synthesis of both GH and Prl and were synergistic in stimulating an increase in GH synthesis in response to triiodothyronine, a physiological regulator of GH synthesis. N'-methylnicotinamide, the most effective agent, was analyzed in detail; it increased the synthesis of both GH and Prl (maximally after 2 days) and increased their mRNAs in parallel; furthermore, this effect was reversible after drug removal. The effects of N'-methylnicotinamide were relatively specific for GH and Prl, since the synthesis of only a few other proteins was affected. These data suggest that changes in ADP-ribosylation can modulate the expression of specific genes.