Flow and conventional DNA-cytofluorometry and in vitro tritiated thymidine cytoautoradiography were employed in the study of the cytokinetic changes induced by Ara-c on the peripheral blood blast cells of six patients with acute non-lymphoblastic leukemias. Nine courses of therapy were studied, in 5 of which Ara-c was administered in low pulse doses (20-60 mg/sqm) repeated every 12 hrs and in the other 4 it was administered in continuous infusion (90-150 mg/sqm) lasting 24 hrs. Low repeated pulse doses have little cytocidal effect and increase the 2n-4n cell percentage and, less markedly, the LI, while the median grain count is reduced. Many U-like cells and accumulation of 3H-TdR labeled blasts in early S phase were observed during therapy. The continuous infusion of Ara-c induces striking cytoreduction and concomitantly decreases the 2n-4n percentage and LI. Progressive recovery of 2n-4n percentage and LI over pretreatment values was observed 24-48 hrs after stopping infusion in one case and was probably due to cell recruitment following tumor mass reduction. Our data agree with the literature, which is briefly reviewed, in that they indicate that Ara-c in a low intravenous pulse frequently dose synchronizes the blasts in S phase while higher doses of the drug are more often cytocidal in S phase and cell recruitment can follow cytoreduction.