Mitotic activity during regeneration in the planarian Dugesia (G) tigrina shows a biphasic pattern, with a first maximum at 4-12 h, a second and higher maximum at 2-4 days, and a relative minimum in between. The first peak is mainly due to pre-existing G2 cells entering mitosis shortly after cutting, whereas the second maximum is due to cells that divide after going through the S period from the onset of regeneration. From a spatial point of view, the highest mitotic values are found in stump (postblastema) regions near the wound (0-300 micron), though regions far from it also show increased mitotic values but always lower overall values. As regeneration continues the postblastema maximum shifts slightly to more proximal regions. In contrast, no mitosis has been found within the blastema, even though the number of blastema cells increases steadily during regeneration. The results suggest that blastema in planarians forms through an early accumulation of undifferentiated cells from the stump to the base of blastema. The results obtained demonstrate that blastema formation in planarians occurs through mechanisms somewhat different to those shown to occur in the classical epimorphic models of regeneration (Annelida, Insecta, Amphibia), and suggest that planarian regeneration could represent an intermediate stage between morphallactic and epimorphic modalities of regeneration.