: epididymides /ɛpɪdɪˈdɪmədiːz/ or /ɛpɪˈdɪdəmɪdiːz/) is an elongated tubular genital organ attached to the posterior side of each one of the two male reproductive glands, the testicles.

The epididymis serves as an interconnection between the multiple efferent ducts at the rear of a testicle (proximally), and the vas deferens (distally).

The major cell types in the epithelium are: The stereocilia of the epididymis are long cytoplasmic projections that have an actin filament backbone.

[4] Because sperm are initially non-motile as they leave the seminiferous tubules, large volumes of fluid are secreted to propel them to the epididymis.

Spermatozoa only reach full motility when inside a vagina, where the alkaline pH is neutralized by acidic vaginal fluids.

Spermatozoa formed in the testis enter the caput epididymidis, progress to the corpus, and finally reach the cauda region, where they are stored.

The epididymis secretes immobilin, a large glycoprotein that is responsible for the creating of the viscoelastic luminal environment that serves to mechanically immobilize spermatozoa until ejaculation.

[9] During emission, sperm flow from the cauda epididymis (which functions as a storage reservoir) into the vas deferens where they are propelled by the peristaltic action of muscle layers in the wall of the vas deferens, and are mixed with the diluting fluids of the prostate, seminal vesicles, and other accessory glands prior to ejaculation (forming semen).

Epididymectomy is also performed for sterilization on some male animals of livestock species so they can be used to detect estrus in females ready for artificial insemination.