The period (per) gene is located on the X chromosome of Drosophila melanogaster. Its expression influences biological clocks in this fruit fly, including the one that subserves circadian rhythms of locomotor activity. Like most X-linked genes in Drosophila, per is under the regulatory control of gene dosage compensation. In this study, we assessed the activity of altered or augmented per+ DNA fragments in transformants. Relative expression levels in male and female adults were inferred from periodicities associated with locomotor behavioral rhythms, and by histochemically assessing beta-galactosidase levels in transgenics carrying different kinds of per-lacZ fusion genes. The results suggest that per contains multipartite regulatory information for dosage compensation within the large first intron and also within the 3' half of this genetic locus.