Mutations at the period (per) locus of Drosophila melanogaster disrupt the circadian rhythm of adult locomotor activity. Molecular studies have shown that this gene is expressed primarily at the embryonic, pupal and adult stages. We have used conditional per mutants to infer the stages of development during which per expression is required for adult rhythmicity. In experiments carried out with germline transformants in which the arrhythmic per01 allele has been transformed with a heat-shock protein 70 promoter-driven per gene (hsp-per transformants) we find that per expression in the adult is both necessary and sufficient for imaginal rhythms. Results obtained with existing per alleles and other per transformant strains that behave as conditional per mutants are consistent with those obtained with these molecularly engineered conditional mutants. Using hsp-per transformants we have found that the per gene product is apparently required only at the time of manifestation of rhythmicity, and can rescue the host's arrhythmic phenotype even when supplied many days after transfer to constant darkness. We present evidence suggesting that it is necessary for pacemaker function itself, rather than being involved in a process that couples the activity of the pacemaker to the output pathway. The levels of per transcript and the abundance and tissue distribution of its protein product observed in hsp-per transformants exposed to different temperature regimes are described. An initial report of some of these results has been published previously (Ewer et al., 1988).