Our laboratory has previously demonstrated that the T-lymphocyte is critical in the development of cyclosporin A-induced osteopenia in the rat model. A similar state of osteopenia is induced by estrogen depletion in the ovariectomized (OVX) rat, which is the animal model of postmenopausal bone loss. However, the role of the immune system, and particularly the T-lymphocyte, in estrogen deplete osteopenia has not been elucidated. We used the Rowett athymic nude rat as our model of T-lymphocyte deficiency. In this study, the experimental rats were divided into four groups as follows: (1) sham-operated Rowett heterozygous (rnu/+) euthymic rats (control group); (2) OVX Rowett heterozygous (rnu/+) euthymic rats; (3) sham-operated Rowett homozygous (rnu/rnu) athymic nude rats, which are T-lymphocyte deficient; and (4) ovariectomized Rowett homozygous (rnu/rnu) rats. Rats were weighed, and venous blood was taken in weeks 2, 4, and 6 for determination of serum osteocalcin. Serum 1,25-dihydroxyvitamin D (1,25(OH)2D) was determined on the day of sacrifice. Following sacrifice, histomorphometry was performed on double-labeled proximal tibial metaphyses. Flow cytometric analysis of splenic mononu-clear cell isolates stained for OX19-positive (CD5) T-lymphocytes was performed. T-lymphocyte analysis revealed significant reductions in both athymic nude groups, while OVX euthymic rats demonstrated a diminished number of T-cells relative to their sham-operated counterparts. Histomorphometric data indicated that both OVX groups exhibited a significant loss of trabecular volume, with associated increases in indices for bone formation and resorption, with resorption likely outstripping formation, resulting in osteopenia. Serum osteocalcin was significantly elevated in the ovariectomized euthymic group throughout the experimental period compared with the control group (p < 0.01); it was elevated in the ovariectomized athymic group on week 4 only (p < 0.01 vs. control). It appears that the T-lymphocyte may not be an essential component in the pathogenesis of estrogen deficiency osteopenia. The contribution of circulating T-lymphocytes as well as other T-lymphocyte-rich organs needs to be explored further.