The distribution, localization, and smooth muscle effects of neuropeptide Y (NPY) were studied in the human female genital tract. High concentrations of NPY immunoreactivity were demonstrated in the uterine artery, the ovary, the fallopian tube, cervix, and the vagina. The NPY immunoreactivity was confined to nerve fibers. The highest density of nerve fibers was observed in relation to blood vessels, although some NPY-immunoreactive nerves were also seen close to nonvascular smooth muscle. The NPY-immunoreactive material throughout the genital tract was identical to synthetic amidated human NPY with regard to size, hydrophobicity, and charge as evaluated by gel filtration, high-performance liquid chromatography, and isoelectric focusing. NPY (10(-10) to 10(-6) M) exerted a direct vasoconstrictory effect on small arteries dissected from the cervix and an additive effect of NPY and norepinephrine responses was observed. Exogenous NPY did not have a direct effect on nonvascular smooth muscle specimens from the fallopian tube or the myometrium. The close relation between NPY-immunoreactive nerves and blood vessels, the presence of NPY-immunoreactive material identical to amidated synthetic human NPY, and the vasoconstrictory effects of NPY indicate that NPY is involved in the regulation of the blood flow in the human female genital tract.