The time course of the early response of amino acid transport and protein synthesis to bovine GH (bGH) administered in vivo was studied in diaphragms of 18-day-old fasted rats. Amino acid transport was assessed by determining the uptake of a-amino-isobutyric acid (AIB), and protein synthesis was determined by measuring the incorporation of phenylalanine and leucine into diaphragm protein in vitro at different time periods after hormone injection. Ten minutes after the injection of bGH iv, both amino acid transport and protein synthesis were stimulated during the subsequent in vitro incubation. The stimulatory effect was dose dependent and had a duration of 60–100 min. The addition of a challenge dose of bGH in vitro (5 μg/ml) or the administration of a second dose of bGH in vivo (50 or 100 Hg) did not stimulate AIB uptake 60 or 180 min after treatment with bGH in vivo, although AIB transport had decreased to the prestimulatory level. Thus, amino acid transport had become refractory to GH. No refractoriness of protein synthesis to bGH in vitro (5 μg/ml) was apparent 60 min after the injection of 50 μg bGH iv. However, a partial refractoriness to bGH in vivo was observed at this time. One hundred and eighty minutes after the injection of 50 μg bGH iv, protein synthesis was totally refractory to the challenge dose of bGH in vitro (5 μg/ml). These results demonstrate that GH in vivo stimulates amino acid transport and protein synthesis in diaphragms of young normal rats. The stimulation is transient, however, and is followed by a period of refractoriness of both amino acid transport and protein synthesis. The findings suggest that changes in responsiveness to GH might play a physiological role in the regulation of protein metabolism in muscle tissue. (Endocrinology106: 291, 1980)