I have measured the effect of hormones and other regulatory factors present in the serum component of the culture medium on the levels of growth hormone and prolactin mRNAs in rat pituitary (GH4) cells. Hybridization of cytoplasmic RNA with growth hormone or prolactin cDNA clones indicate that serum depletion reduces significantly the amount of these two mRNAs. The localization of these two genes in chromatin was also analysed using micrococcal nuclease as a probe. At intermediate levels of digestion (about 10% of the input A260 released into a soluble supernatant S1), the bulk of both growth hormone and prolactin genes are rapidly solubilized by the nuclease and appear in the soluble supernatant S1. Nevertheless, at low levels of digestion (less than 4% of the input A260 released into S1) the growth hormone gene remains exquisitively sensitive to micrococcal nuclease while the sensitivity of the prolactin gene is reduced considerably. When one compares the distribution of growth hormone and prolactin genes in chromatin fractions differing in nuclease sensitivity and derived from cells grown in control medium or in depleted medium, it appears that markedly reduced transcriptional activity of the prolactin gene shows no correlation with altered chromatin structure. On the other hand, the chromatin structure of the growth hormone gene is significantly altered when transcription is markedly reduced.