Transcription of GH receptor (GHR) mRNA is initiated from multiple promoters. Most GHR mRNA arise from GHR Promoter 1 (GHR P1) and GHR P2, which transcribe GHR 1A and GHR 1B mRNA, respectively. Our objective was to characterize the expression of GHR 1A and GHR 1B mRNA in liver of neonatal intact (1 d of age) and castrated (14, 28, and 42 d of age) male pigs (Exp. 1; n = 6 per age group), intact male pigs treated with recombinant porcine ST (rpST) or control (Exp. 2; 21, 42, and 77 d of age; n = 4 pigs per treatment per age), and pregnant gilts treated with rpST (n = 6) or control (n = 7) (Exp. 3). Tissue samples were collected at slaughter for mRNA analyses. The porcine GHR 1A and GHR 1B cDNA were cloned and were homologous to GHR cDNA isolated from other species. Ribonuclease protection assays were used to measure GHR 1A and GHR 1B mRNA. Liver expressed GHR 1A and GHR 1B mRNA, whereas muscle, uterus, and ovary expressed GHR 1B mRNA. The GHR 1A mRNA in the liver of neonatal intact and castrated male pigs (Exp. 1) was expressed at very low levels on d 1, 14, and 28, and two of six pigs expressed a high level of GHR 1A on d 42. The GHR 1B mRNA, however, was detected at all ages (d 1 through 42), and the amount of GHR 1B increased (P<.05) on d 42. The liver of intact male pigs (Exp. 2) expressed GHR 1B mRNA by 21 d, whereas a high level of GHR 1A mRNA was not detected until d 42 (P<.10). Administration of rpST had no effect on expression of GHR 1A or GHR 1B mRNA in pigs younger than 77 d (Exp. 2), but it tended to decrease (P<.10) GHR 1A mRNA but not GHR 1B mRNA in pregnant gilts (Exp. 3). In conclusion, GHR mRNA in porcine liver was composed of at least two variants (GHR 1A and GHR 1B). The GHR 1B mRNA was the major GHR mRNA in pig liver before 77 d of age. The GHR 1A mRNA increased after 42 d of age and tended to undergo specific down-regulation in response to rpST in pregnant gilts.