We report the complete primary structures of two major chicken non-ribosomal nucleolar proteins known as nucleolin/C23 and NO38/B23, respectively. By comparison with homologous proteins from other species, this sequence information contributes to the identification of evolutionarily conserved motifs that may be relevant to the function and subcellular distribution of the two proteins. Using cDNA probes and monoclonal antibodies, we have also studied the expression of nucleolin and NO38 in the course of chicken embryogenesis. In all tissues examined, Northern analyses revealed single hybridization signals for nucleolin (at 3.0 kb) and NO38 (at 1.6 kb), and no evidence was obtained for multiple protein products. In total embryos between days 3 and 11 after egg laying, nucleolin and NO38 mRNA and protein levels decreased in parallel (2-5-fold), suggesting transcriptional down-regulation of expression. Coordinate expression of nucleolin and NO38 was observed also when examining individual tissues at various stages of development. Interestingly, however, there was no consistent correlation between relative mRNA and protein levels. In particular, several adult tissues contained exceedingly low levels of either nucleolin or NO38, despite the presence of large amounts of corresponding mRNAs. From these results we conclude, first, that the expression of nucleolin and NO38 is controlled coordinately, and, second, that regulation is likely to involve both transcriptional and posttranscriptional mechanisms.