The proximal promoter of the human growth hormone gene (GH1) is highly polymorphic. We tested if promoter haplotypes differing at possibly functional sites, namely -278T/G (in the NF1 binding site), -75A/G (in the proximal Pit-1 binding site) and -57G/T (in the VDR binding site), induced a different luciferase activity when transfected in a rat pituitary cell line. The presence of a G instead of an A at position -75 induced a more than two-fold reduced activity (p<0.0001). In accordance with this findings the electrophoretic mobility shift assay demonstrated a reduced affinity of the -75G for the pituitary transcription factor Pit-1. Despite the strong effect of this polymorphism in vitro, the -75G variation was not associated to an impairment of the GH secretion in vivo.