In mammalian nuclei, precursor messenger RNA splicing factors are distributed non-uniformly. Antibodies directed against structural polypeptides of small nuclear ribonucleoprotein particles (snRNPs) and some non-snRNP splicing factors have shown that these components are concentrated in about 20-50 nuclear 'speckles'. These and other non-homogeneous distributions have been proposed to indicate nuclear 'compartments' that are distinct from the sites of transcription and in which RNA processing occurs. We have tested this idea using a new approach. Previous structural and biochemical data have shown that splicing can occur in association with transcription. Nascent RNA of specific genes can be detected by in situ hybridization as intense spots of nuclear stain which map to the sites of transcription. Here we identify active pre-mRNA splicing sites by localizing the nascent spliced mRNA of specific genes. We find that splicing occurs at the sites of transcription, which are not coincident with intranuclear speckles. We conclude that the nucleus is not compartmentalized with respect to transcription and pre-mRNA splicing.