Male Long-Evans rats 35-40 days of age were deprived of food for a 2 day period and then were refed ad lib. Growth rates of body weight and of tail length decreased during the fast; with refeeding catch-up growth acceleration occurred. Sulfate uptake by rib cartilage in vitro was reduced during fasting, but during recovery it returned to the control value at 7 days and remained normal thereafter. In vivo sulfate uptake was reduced during the fast and returned to the control value by recovery day 14; at recovery day 21 it exceeded the control value but by recovery day 28 there was again no significant difference between fasted and control values. Somatomedin (SM) activity in serum decreased during the fast; it returned to the control value by recovery day 7. The results show a correlation between in vitro sulfate uptake by rib cartilage during late recovery periods and catch-up growth. Parallelism in the dose-response to normal rat serum in in vitro sulfate uptake may also correlate with catch-up growth capability. Recovery of in vitro sulfate uptake seems to correlate with previously reported ultrastructural findings in cartilage. Catch-up growth apparently is limited in certain circumstances because of abnormalities of cartilage structure and function.