We have investigated the effects of leaf and bark decoctions of Ocimum gratissimum, Azadirachta indica, Morinda lucida and Enantia chlorantha on (a) the course of Plasmodium yoelii nigeriensis malaria (b) reticulocyte and haematocrit values and (c) nucleated cell numbers in the spleen, bone marrow, peritoneum, liver and peripheral blood of Swiss albino mice. Results obtained showed that normal mice infected with the parasite (10(4)/mouse) suffered fulminant parasitaemia which resulted in death, 7-10 days later. All infected mice treated with chloroquine survived. On the other hand all infected mice treated with the medicinal plants exhibited varying percentages of chemosuppression of early parasitaemia which did not lead to their survival. The total number of nucleated cells in the liver, spleen and peripheral blood of malaria-infected mice increased enormously before the animals died. Such increases were maintained in other groups of mice treated with the medicinal plants. In the non-infected mice, O.gratissimum and E. chlorantha administration increased the number of nucleated cells in the spleen, liver and peripheral blood. Chloroquine on the other hand decreased the number of nucleated cells in both the malaria-infected and un-infected mice. There was also a decrease in reticulocyte numbers in the blood of normal mice injected with chloroquine. Conversely reticulocyte numbers increased in normal mice administered with some medicinal plants. Acute and chronic toxicity tests revealed that some of the medicinal plants were much more toxic than others.(ABSTRACT TRUNCATED AT 250 WORDS)