Cycloheximide and chloroamphenicol, specific inhibitors of protein translation in the cytoplasmic and cyanellar compartments, respectively, of Cyanophora paradoxa, have been employed in 30 min pulse-labelling experiments by using [NaH-14C]O3 to label total cell proteins in vivo. Cyanellae purified from host cell lysates were separated into soluble and thylakoid fractions and analysed by one- and two-dimensional polyacrylamide gel electrophoresis (PAGE) to determine the distribution of radioactivity in the cyanellar polypeptides. Analysis of the autoradiograms of electrophoretically resolved proteins of the cyanellae indicates that about 70% of the total number of cyanellar proteins visualized in the controls are synthesized on cytoplasmic ribosomes. The majority (81%) of the soluble cyanellar proteins appear to be cytoplasmically synthesized. In contrast, the majority (70%) of the thylakoid proteins are synthesized within the cyanellae. The observations also suggest that the polypeptides synthesized within the cyanellae include species that are the most abundant and rapidly turned over. A number of the polypeptides previously identified have now been characterized with regard to their sites of synthesis. In addition, we report on labelling experiments involving rifampicin, a specific inhibitor of cyanellar transcription, which indicate that different mRNAs within the cyanellae have markedly different stabilities.