Thymidine kinase (TdR kinase) activity has been measured at various stages of regeneration in the planarian Dugesia dorotocephala. It is argued that TdR kinase activity is a valid, indirect measurement of cell proliferation in this organism. The earliest measured responses to wounding and the initiation of regeneration show maximal TdR kinase activities in non-blastema regions. Later phases of regeneration show a shift of maxima to the blastemata with the general relations: prospective head blastemata have higher activity than prospective tail blastemata and posterior blastemata lag behind more anterior blastemata of the same developmental fate. It is concluded that the earliest proliferative response to wounding is in non-blastema regions with secondary waves of proliferation in the blastemata proceding differentiation. The data are consistent both with models invoking long-range cell migration and with models of local origin of the blastemata. Hydroxyurea treatment increases the level of TdR kinase activity and at low concentrations causes a modest acceleration in the rate of regeneration. Hydroxyurea treatment also produces bipolar (hetermorphic) regenerants from posteriorly-derived sections. The possible role of cell number is invoked in relation to bipolar forms and as a regulator of normal differentiation.