The effect of nitrogen nutrition on the accumulation of seed storage proteins has been studied in vitro by cultivating on agar media maize (Zea mays L.) endosperm explants from seeds at 10 days after pollination. The experiments were performed on various genetic backgrounds bearing different opaque2 (o2) mutant alleles and on the corresponding wild-type lines. In the seed of the o2 genotypes the high molecular weight alpha-zein polypeptides (zHs), whose transcription is Opaque2 (O2) regulated, are absent or extremely reduced. The endosperms were incubated on basal agar medium with amino acid supply. In these growth conditions, fresh and dry weights increased in both wild-type and o2 endosperms, irrespective of the genetic background. In 4 out of the 5 o2 mutant genotypes analysed we detected an accumulation of the zHs similar to the corresponding wild-type explants or seeds. However, in one of these mutants, Mo17o2R, the addition of amino acids to the culture media had no effect on the zH accumulation. We showed that the Mo17o2R behaviour is not due to a negative regulation but to the absence of putative transcription factor(s) able to regulate the zH transcription occurring in the other o2 mutants.