Two types of storage protein, vicillin and legumin, occur in the developing pea seed. Storage-protein gene expression has been studied during cotyledon development by assaying specific transcripts produced by nuclei isolated at different stages, and from pea leaves. The proportion of vicilin to legumin transcripts changed during development: vicilin transcripts predominated at 9 and 11 days after flower opening (d.a.f.) and were similar in amount to legumin at 14 d.a.f., whereas at 18 d.a.f., legumin transcripts predominated and little vicilin transcription was observed. The rate of storage-protein transcription correlated with previously determined (Gatehouse et al. 1982) mRNA levels during seed development; these transcripts were not detected in similar assays using leaf nuclei. Transcription by cotyledonary nuclei for short times indicated that post-transcriptional processing may be a factor in regulating mRNA levels, at least in the earlier part of seed development.