The male accessory gland of Drosophila is an adult secretory tissue which contributes many products to the male ejaculatory fluid. The secretions of the accessory gland affect the behavior and physiology of the female fly after mating, reducing her receptivity to courtship and stimulating egg production and oviposition. We have examined the developmental and mating-stimulated expression of two accessory gland proteins in the male and their transfer to and fates in the mated female. One of these proteins, msP 355a, has features of a prohormone and contains a region with amino acid sequence similarity to the egg-laying hormone of Aplysia; the other, msP 355b, is a small acidic protein. Both proteins are first detected in the accessory gland only after eclosion, although their transcripts are already present in late pupae. Both proteins are initially detected in the two morphologically distinct secretory cell types of the accessory gland, the main cells, and the secondary cells. In the glands of aged virgin males, they are only detected in the large vesicles of the secondary cells and in the lumen of the gland. Copulation results in an increase in the mRNAs for both proteins, as well as renewed translation of the proteins at least in the main cells. Both proteins are transferred to the female genital tract during copulation, and rapidly enter the female hemolymph. msP 355a is subject to rapid and specific cleavage within the female genital tract, but not in the hemolymph; msP 355b is not cleaved in either the female genital tract or the hemolymph.