The occurrence of osteoplastic activity in adult osteocytes from the femoral cortex of laying hens previously kept on a hypocalcaemic diet was studied after return of the animals to a calcium-rich alimentation. The following techniques were used: 1) tetracycline labelling of new calcifying matrix; 2) osteoid staining with alcian blue at pH 1.9 or toluidine blue at pH 3.1; 3) electron microscopic observations; 4) autoradiographic evaluation of (3H) proline uptake by osteocytes. Quantitative evaluation of (3H) proline uptake by osteocytes. Quantitative evaluation of the preparations consistently showed indication of osteoplastic activity in at least 20% of the osteocytes with relatively higher values in the metaphysis than in the diaphysis.