Using a novel marker of C-type natriuretic peptide (CNP) synthesis (NT-proCNP) we have recently shown that plasma NT-proCNP is strongly correlated with skeletal growth and markers of bone formation and is reversibly reduced by glucocorticoids. The effects on CNP of other catabolic or anabolic factors, known to affect skeletal growth, are unknown. Accordingly we have studied the response of plasma CNP forms to acute catabolic (caloric restriction) and anabolic (growth hormone (GH) stimulation) interventions in lambs and related the findings to circulating IGF-1 levels, growth velocity and markers of bone formation. Lambs fed a reduced caloric intake (25% of normal) for 6 days exhibited reduced live-weight, plasma urea and IGF-1 (p<0.001 for all) compared with control lambs. Basal levels of NT-proCNP (40.1 +/- 0.9 pmol/L) fell promptly to a nadir (28.1 +/- 0.8 pmol/L, p<0.001) on day 6, returning rapidly to basal levels upon re-feeding. Although plasma ALP fell (p<0.001) reductions in metacarpal growth velocity were not significant within the 12 day period of study. In contrast to caloric restriction, long-acting bovine rGH (2.5mg/kg on days 0 and 6), as expected, increased plasma IGF-1 more than 2 fold above control for 12 days (p<0.001). Growth velocity did not differ during the 30 days of observation, and consistent with unchanged growth velocity, plasma NT-proCNP and ALP were also unaffected. In conclusion, CNP synthesis and markers of bone formation are acutely sensitive to catabolism but unaffected by doses of GH that fail to stimulate skeletal growth.