Plasma somatomedin-C (SM-C), important for linear growth, is reduced under conditions of caloric deficit whether induced by diet or exercise. The purpose of this study was to determine the effect of diet and exercise on plasma SM-C levels in 37 8- to 10-year-old children, 18 swimmers (SWM) and 19 control children (CTL). All children kept a 3-day dietary record for 2 weeks prior to collection of a fasted, resting blood sample. Both groups showed RDA-normal total kcal intake (SWM = 2301 ± 664; CTL = 2072 ± 449) and twice the RDA for protein for this age group (SWM = 3.1 ± 2.3 g • kg-1; CTL = 3.0 ± 2.4 g • kg-1). Plasma SM-C levels for SWM (1.39 ± .84 U • ml-1) and CTL (.91 ± .33 U • ml-1) were significantly different (P<.01) but within the normal range for this age group. These data suggest that 8- to 10-year-old children who are training regularly, in combination with adequate energy and protein intake, show SM-C levels within the normal range expected for this age.