Autoradiography was used to study the incorporation of tritium-labelled nucleic acid precursors into the sporogonic stages of Plasmodium cynomolgi in Anopheles b. balabacensis. Infected mosquitoes were fed on either 3H-adenine (5, 25 and 50 muCi/ml or 3H-thymidine (50 muCi/ml) for various periods after the blood meal. 3H-adenine was incorporated into DNA and RNA of the developing oocysts and the resulting sporozoites. Midgut epithelial cells incorporated label from 3H-adenine into nuclear and cytoplasmic regions. In both parasite and host tissue RNA-label was removed after RNase treatment. 3H-thymidine was not taken up by the developing oocysts or sporozoites while incorporation into the cell nuclei of the adjacent midgut epithelium and fat-body of the mosquito was shown. The observed incorporation of 3H-adenine, but not 3H-thymidine, into the sporogonic stages of Plasmodium supports the assumption that the malaria parasite needs exogenous sources of purine but relies on the de novo synthesis of pyrimidine during its development in the mosquito.