Fifty-seven calves from two lines selected for high (n= 36) and low (n= 21) milk fat yield were typed for a known restriction fragment length polymorphism (RFLP) in the growth hormone (GH) gene. The genetic origin of the RFLP is an insertion (I)/deletion(D) in the 3′-region of the growth hormone gene. The calves were tested at 3–4 months of age and again at 9–10 months of age for plasma GH concentration before and following three GH release tests: fasting-refeeding (n= 57), arginine injection (n= 28) and thyrotropin-releasing hormone (TRH) injection (n= 24). The RFLP typing showed 36, 19 and 2 calves of the genotypes II, ID and DD at the I/D site, respectively. The frequency of the D-allele was higher in the high than in the low selection line (0.29 vs. 0.05,P<0.01). At 9–10 months of age the GH release following TRH injection was larger in calves with genotype II (35.9 ± 4.5 ng/ml) compared with calves with genotype ID (23.6 ± 3.8 ng/ml) (P<0.05), and the calves from the high line had a higher GH release after TRH injection (42.9 ± 5.5 ng/ml) compared to low-line calves (19.7 ± 3.4 ng/ml) (P<0.001). These results may indicate that the I/D site may affect or be a genetic marker for TRH-induced GH release in dairy calves.