Long-term administration of GH to normal, well-fed lambs caused a significant increase in net muscle growth. This could be accounted for by increased rates of muscle protein synthesis, although red and white muscles responded differently. The increased rate of protein synthesis was due to an increased protein synthetic capacity (increased muscle RNA content) but efficiency per unit of RNA also tended to increase in red muscle. For similar increases in net growth protein turnover was increased to a much greater extent in red than in white muscle. The ratio of collagen to non-collagen protein was unaffected in both muscle types by GH treatment, even though collagen synthesis rates were significantly increased in red muscle. To date, GH is the only anabolic agent in ruminants which acts via increased rates of protein synthesis rather than by decreased rates of protein degradation.