Diurnal variation of mitosis and tritiated thymidine ([ 3H]-T) labelling of the normal unstimulated 6–8-week-old rat periodontal ligament (PDL) was determined under standardized conditions. Periodicity was observed with minimal [ 3H]-T labelling and mitosis at 9 p.m. and maximal levels at 9 a.m. and 3 p.m. respectively. The kinetic nature of the periodontal ligament, and the mechanism of osteoclast cytodifferentiation were investigated by using parathyroid hormone (PTH) to accelerate a slowly proliferating population of osteoclast-producing PDL cells and measuring initial kinetics of [ 3H]-T uptake, mitosis, and accumulation of osteoclasts. Simultaneous increases, beginning 1–3 hr after PTH, of labelled cells, mitotic figures, and total number of osteoclasts indicated that at least three separate populations of cells were present initially: (1) G 1 blocked; (2) G 2 blocked, and (3) partially differentiated pre-osteoclasts. Reviewing available evidence it was concluded that: (1) morphologically indistinguishable PDL cells are a mixed population of relatively undifferentiated cells, as well as cells along separate pathways of cytodifferentiation to osteoblasts, osteoclasts and cementoblasts; (2) cells within the PDL (pre-osteoclasts) and/or cells migrating in are the source of the initial wave of osteoclasts observed within 9 hr after PTH; and (3) PTH stimulation of the proliferation-differentiation pathway to osteoclasts involves a general, probably metabolic, effect on all cells in the sequence.