Although sheep with a relatively high fleece weight have reduced fat deposition and a lower reproductive rate, research has failed to identify any specific metabolic demand that growing a fleece places on the physiology of the sheep. This paper reports two experiments in which the effect of fleece weight was examined in ewes infused intravenously with amino acids. The first experiment was carried out when the ewes were 107 days pregnant, and the second 2 years later in non-pregnant ewes from the same flock. The ewes were derived initially from groups that differed in staple strength, but there was no effect of staple strength group on the characteristics measured in either experiment. In Expt 2, ewes were also infused with a bolus of L-[ring-d(5)] phenylalanine, and the enrichment in plasma determined by GC/mass spectrometry over the next 24 h. In both experiments, fasting plasma insulin concentrations were lower (P<0.05) in ewes with a high fleece weight, and this difference continued during infusion in Expt 2 (P<0.05). In Expt 1, infusion of ewes with amino acids resulted in higher (P<0.05) plasma concentrations of α-amino nitrogen ( indicating amino acids) in the ewes with a higher fleece weight, and in Expt 2, ewes with a high fleece weight had a 19% higher rate of appearance of endogenous phenylalanine (P<0.05). We conclude that sheep with high wool growth rates have higher whole-body protein turnover rate, which may be achieved in part by lower insulin concentrations. Lower insulin in turn provides a mechanism through which wool growth rate may influence energy availability to other tissues.