Protocollagen proline hydroxylase (PPH) activity in the course of rebound of rat carrageenin granuloma was assyed to investigate its relationship with collagen blosynthesis. The specific activity of PPH which was inhibited significantly by treatments with hydrocortisone acetate 93mg/kg/day, 3days) was recovered near to the normal level by 48hr after the cessation of the corticoid treatments. The total enzyme activity of the granuloma of treated group was not yet recovered even on the 3rd day, still showing a significant difference from control. PPH activity expressed in terms of granuloma cells between control and the drug treated group, however, was changed showing no apparent difference throughout the experimental period. These results suggested that the synthesis of protocollagen(collagen precursor) rather than PPH activity was directly affected by administration and withdrawal of corticoid.