In Snell dwarf mice, the influence of short-term treatment with human growth hormone (hGH) or thyroxine on the proliferative and sulphation activity of the proximal tibial growth plate was studied. By autoradiographic methods, the [3H]methylthymidine incorporation after a single injection was measured, after 2 hr incorporation time. The labelling index was calculated and the number of labelled mitoses was counted. In addition, the distribution of the labelled nuclei over the proliferating and degenerating zones was determined by continuous labelling for 25 and 73 hr. In untreated dwarf mice after [3H]-methylthymidine administration, the number of labelled nuclei in the growth plate is low. Labelling occurs, as expected, mainly in the cells of the proliferative zones. The number of labelled nuclei in control dwarf mice was similar after 25 and 73 hr continuous labelling. This suggests that many cells are in a resting G0 or prolonged G1 phase. Both hGH and T4 treatment induce a significant increase of the number of labelled nuclei per growth plate and of the number of mitoses. Since hormonal treatment induces a small number of mitoses after 2 hr incorporation of the label, the minimal G2 phase of the cell cycle is less than 2 hr. In addition, treatment with hGH and T4 stimulates chondrocytes in the zone of proliferative and hypertrophic cells to actively incorporate [35S]-sulphate.