In the femoral diaphysis of 31-day-old rats given tetracycline daily for varying periods up to 17 days, there was a progressive increase in osteocytic perilacunar tetracycline label width as a function of labeling time. That this represented osteocytic bone formation (i.e., matrix formation and mineralization) was confirmed by 1) an increase in labeling of perilacunar matrix with procion as a function of labeling time and 2) a decrease in lacunar area with osteocyte age. Perilacunar bone apposition (width of new bone bone added per day) was maximum in newly formed osteocytes (0.22 μ/day), and subsequently, there was an exponential decrease in this rate as a function of osteocyte age. Although this study clearly demonstrates that osteocytes form bone, the total volume of bone formed in cubic millimeters per day in a 1-mm-thick section of the femoral diaphysis was only 0.0012 for osteocytes as compared with 0.088 for osteoblasts. Since osteocytic bone formation in the entire skeleton would have to increase by about 1,600% to decrease serum calcium by 1 mg/100 ml in 1 hr, this process probably does not play a major role in regulating serum calcium.