We investigated the utriculus prostaticus from the fetal period up to adulthood in 148 prostates. During the second half of gestation the utriculus had a simple tubular or a cystic form and was lined with metaplastic squamous epithelium which showed immunohistochemical positivity for different keratins, carcinoembryonic antigen, and peanut agglutinin binding sites. After birth, alveolar outgrowths of the utriculus developed. After puberty, the utriculus had become a complicated and variable structure. The epithelium no longer differed from that of the prostate glands either morphologically or immunohistochemically. Within the epithelium numerous endocrine cells were found containing neuron-specific enolase, chromogranin, and serotonin. The utriculus and ejaculatory ducts were embedded in a fibrous stroma with, after birth, numerous plexus-like blood vessels. This fibrous zone was peripherally bordered by a layer of smooth muscle. There was no evidence for a function of the utriculus differing from that of the prostate glands. Since the epithelium of both structures is identical immunohistochemically, the epithelium of the sinus urogenitalis most likely particpates in the lining of the utriculus during embryogenesis.