The oxazolone-induced response in the paracortex of draining lymph nodes is characterized by an early increase in the proliferative activity that decreases to control levels when stimulation is continued. The possibility that this may be a toxic side effect of the concentrated oxazolone solution used was investigated by simultaneous registration of the changes taking place in the thymus. These were found to be different from toxin-induced changes and compatible with cell loss due to massive emigration of cells. Repopulation of the thymus took place over the last 1 1/2 week of stimulation. It was concluded that the changes in the thymus as well as the decline of the proliferative activity in the paracortex, are most likely physiological responses. The most important factor in maintaining a high production of paracortical lymphocytes under chronic stimulation is the increase in the lymphocyte mass in the paracortex.