We have found previously that the sequences important for recognition of pre-mRNA introns in dicot plants differ from those in the introns of vertebrates and yeast. Neither a conserved branch point nor a polypyrimidine tract, found in yeast and vertebrate introns respectively, are required. Instead, AU-rich sequences, a characteristic feature of dicot plant introns, are essential. Here we show that splicing in protoplasts of maize, a monocot, differs significantly from splicing in a dicot, Nicotiana plumbaginifolia. As in the case of dicots, a conserved branch point and a polypyrimidine tract are not required for intron processing in maize. However, unlike in dicots, AU-rich sequences are not essential, although their presence facilitates splicing if the splice site sequences are not optimal. The lack of an absolute requirement for AU-rich stretches in monocot introns in reflected in the occurrence of GC-rich introns in monocots but not in dicots. We also show that maize protoplasts are able to process a mammalian intron and short introns containing stem--loops, neither of which are spliced in N.plumbaginifolia protoplasts. The ability of maize, but not of N.plumbaginifolia to process stem--loop-containing or GC-rich introns suggests that one of the functions of AU-rich sequences during splicing of dicot plant pre-mRNAs may be to minimize secondary structure within the intron.