Membrane associated GDP kinase activity increases in cultured normal rat kidney cells treated with 2-pyridine carboxylic acid. This increase is associated with an augmented agonist activation of adenylate cyclase. Known components of adenylate cyclase are unaffected by drug treatment. Results supply experimental evidence for the proposal that GDP kinase is an essential component in regulation of adenylate cyclase activity. Syntheses of growth hormone and prolactin by cultured rat pituitary GH3 cells are increased by treatment of cells with inhibitors of ADPribosylation. Growth hormone synthesis is increased synergistically by treatment with both ADPribosylation inhibitors and thyroid hormone, a physiological stimulator of growth hormone synthesis. mRNA contents increase in parallel with the increases in protein syntheses. Results point to a role for ADPribosylation in regulation of gene transcription.