It is known that marrow fat content increases in a variety of osteoporoses. This study sought to clarify this phenomenon by combining bone and marrow histomorphometry, and to determine (1) whether the marrow fat increase follows or precedes diminishment of bone volume, and (2) whether the increase in fat volume is due to adipocyte growth or proliferation. The relationship between marrow fat content and bone turnover was studied in the metaphysis and epiphysis of the proximal tibia in 20 Ovariectomized and 20 intact 200 gm Sprague-Dawley rats. The results were examined after one and three months. The epiphysis had greater trabecular bone volume than the metaphysis. The metaphysis exhibited a decrease in bone volume with time; the epiphysis did not. Following ovariectomy, the bone volume diminished twice as much in the metaphysis as in the epiphysis. In the epiphysis, as bone volume fraction fell following ovariectomy, the vacated space was filled by both hemopoietic and adipose tissue. In the metaphysis, space-filling was time-dependent: hemopoietic tissue at one month and adipose tissue at three months. Marrow fat content increased with both time and ovariectomy in the metaphysis, but only with time in the epiphysis. Thus, marrow fat increased after bone volume began to decline in the metaphysis, and not at all in the epiphysis. Ovariectomy increased erosion surface in both the epiphysis and the metaphysis, but bone formation rate was increased only in the epiphysis. There was a reciprocal relationship between marrow fat content and bone formation rate.