GH and IGF-I plasma concentrations were measured in lambs during an acute phase response induced by an intrathoracic injection of yeast. The acute phase response was indicated by reduced feed intake, weight loss and an increase in plasma concentrations of the acute phase protein haptoglobin. Intensive blood sampling on day 1 revealed elevated basal concentrations of GH in the yeast-injected group compared with concentrations in pair weight and ad libitum fed control lambs. This suggests that at the beginning of an acute phase response there is an increase in either GH secretion or the half life of GH. No evidence of a specific GH-binding protein in sheep plasma could be detected. IGF-I concentrations in the yeast-injected group remained constant for 3 days then increased to a peak level at day 6. In contrast, plasma IGF-I concentrations were depressed from days 3 to 6 in the pair weight control group and they were unchanged in the ad libitum fed controls. When the IGF-I concentrations were elevated in the yeast-injected group, this group had a higher daily weight gain despite their lower feed intake compared with the ad libitum fed controls. These results suggest that IGF-I may be associated with the increase in weight in the late stage of an acute phase response during recovery from an infection or injury.(ABSTRACT TRUNCATED AT 250 WORDS)