The synthesis of influenza virus ribonucleoprotein structures (RNPs) in infected chick embryo cells was analysed by polyacrylamide gel electrophoresis (PAGE) in the presence of sodium deoxycholate which resolves the RNPs into five size classes. A relatively small proportion of total RNPs accumulated in the nucleus but free NP protein was found there in large amounts over the period 1.5 to 4 h post-infection. In contrast, by 4 h post-infection, all cytoplasmic NP was complexed into RNP structures. At early times, during at 15 min pulse of (35S]methionine, nearly all the newly synthesized NP was incorporated into RNPs but by 4 h the majority of pulse-labelled NP was present as free protein. However, the proportion of free NP: NP in RNPs remained constant over the 1.5 to 4 h post-infection period, indicating that there was a delay before the NP synthesized later in infection was assembled into RNP structures. Individual RNP size classes were predominantly cytoplasmic and accumulated at similar rates but were not produced in equimolar amounts. The rates of synthesis of individual RNPs were in general agreement with their rates of accumulation with the remarkable exception of RNP d (containing RNA 7, the matrix protein gene). This was synthesized nearly 10-fold faster but accumulated at the same rate as the other RNPs. Possibly RNP d is more rapidly degraded than the other RNPs.