The adenovirus L1 unit represents an example of an alternatively spliced precursor messenger (pre-mRNA) where on 5' splice can be jointed to one of two alternative 3' splice sites, producing the 52,55K or the IIIa mRNAs (Fig. 1a). Efficient usage of the distal IIIa 3' splice site requires late viral protein synthesis and is therefore confined to the late phase of virus infection. Here we show that, in extracts from uninfected cells, the classical SR proteins, which are essential splicing factors, inhibit IIIa pre-mRNA splicing by binding to an intronic repressor element and preventing recruitment of the U2 small nuclear ribonucleoprotein particle to the spliceosome. We further show that the viral repressor element has splicing-enhancer activity when appropriately placed in the pre-mRNA. Together, our results demonstrate that SR proteins function as activators or repressors of splicing depending on where on the pre-mRNA they bind.