Female Wistar-Furth rats were implanted sc with GH3 rat pituitary tumor cells. Tumors were palpable by 4 weeks, and animals were killed periodically from 5–9 weeks. Tumor-bearing rats (n = 10) were heavier than their respective controls, reaching a weight of 372 ± 3 by 9 weeks vs. 195 ± 5 g in controls (mean ± SE). Circulating serum GH levels increased in tumor-bearing animals from 218 ± 50 to 9067 ± 962 ng/ml. Serum insulin-like growth factor I (IGF-I) levels were elevated 3-fold in tumor-bearing rats. After death, pituitary glands were excised, and their total RNA was extracted. GH mRNA was assayed by dot hybridization of immobilized pituitary RNA with [³²P]cDNA for rat GH. The hybridization signal was quantified by densitometry of autoradiographs. Pituitary rat GH mRNA levels were suppressed 50% in tumor-bearing animals after 5 weeks. By the end of the 9-week period, pituitary GH mRNA levels were undetectable in tumor-bearing animals. The results show that GH tumor-bearing animals exhibit high levels of circulating GH and IGF-I and suppressed endogenous pituitary GH mRNA levels. This may be caused by autoregulation of pituitary GH gene expression either at the level of the hypothalamus or by a direct effect of GH on the pituitary. Alternatively, the elevated levels of IGF-I may be responsible for the suppression of pituitary GH gene expression. (Endocrinology118: 915–918, 1986)