Plasma glucose, immunoreactive somatomedin-C (Sm-C), GH, insulin (IRI) and T3 were measured daily in 20 obese subjects before and during a 9-day fast. Control levels of Sm-C were normal and exhibited no significant correlation with any of the other parameters investigated. During fasting there were no consistent variations in GH, but significant decreases occurred in blood glucose, IRI, T3 and Sm-C levels. While individual Sm-C patterns varied widely, as a group the fasting Sm-C values showed a positive correlation with the fasting IRI and T3 levels, and with control body weight. These findings suggest that the decrease in Sm-C which occurs during caloric deprivation may be the result of concomitant decreases occurring in T3 and IRI levels.