The incorporation of 14C-labeled amino acids and of [ 3H] uridine into the chloroplast ribosomes of Acetabularia has been investigated in the presence of chloramphenicol and cycloheximide. Cycloheximide inhibits both uridine and amino acid incorporation. Control experiments show that this inhibition cannot be explained by an inhibition of total chloroplast RNA synthesis by cycloheximide. Chloramphenicol has a stimulatory effect on uridine as well as on amino acid incorporation. Such results indicate that the proteins of at least the 44S subunit of chloroplast ribosomes are synthesized on the 80S ribosomes. This process appears to be closely related to the synthesis of chloroplast ribosomal RNA.