Psychotria forsteriana A. Gray is a tropical plant endemic in the Vaté Islands. Previous studies have shown that an alkaloidal extract of its leaves is highly cytotoxic on cultured rat hepatoma cells (HTC), and the isolated polyindolinic alkaloids (formed by condensation of several N-methyltryptamine units) are also cytotoxic on rat hepatoma HTC and mice leukemic L1210 cell lines. As part of the pharmacognostical study of this genus, this study aimed to complement the cytotoxic properties of this species by testing the biological activity of alkaloid extracts from different organs (leaves: PFF, rootbark: PFER, stem bark: PFET, fruits: PFFr) on human leukemic (Molt4) and rat hepatoma (FAZA) cell lines, and some isolated alkaloids. Cell growth or survival was measured by 3H-thymidine incorporation, and FAZA cells were observed microscopically after GIEMSA coloration. Alkaloid extracts were obtained via classical acid-base extraction, and alkaloids were isolated by column chromatography and semi-preparative HPLC, identified based on spectral data. Results showed that all alkaloid extracts from different organs exhibited cytotoxicity on Molt4 cells: doses above 10 μg/ml caused 100% cell mortality, and they remained active at 2 μg/ml (with PFER showing 30% residual cells at 2 μg/ml). Microscopic observation of FAZA cells after GIEMSA coloration confirmed cytotoxicity, with cell lysis at 10 μg/ml. Isolated alkaloids were polyindolinic, consisting of 2-5 N-methyltryptamine units, and their activity correlated with structural features (relative molecular mass, polymerization mode, stereoisomerism), with higher molecular mass alkaloids being more active. Molt4 cells were more sensitive than HTC cells, requiring lower doses for 100% mortality, and the 3H-thymidine incorporation method was convenient, rapid, and sensitive, consistent with classical microcytometer counting. These alkaloids were cytotoxic in vitro in the micromolar range to both cell lines. Preliminary results confirmed the cytotoxic effects of polyindolinic alkaloids on human and rat tumor cell lines. Their activities depend on structure and dose, and further stereochemical studies will clarify structure-activity relationships. These alkaloids may be potent cytotoxic antitumoral agents in vitro.