most primitive of the three vertebrate kidneys, active in the adults of some primitive fish (lampreys and hagfish), the embryos of more advanced fish, and the larvae of amphibians. It is a paired organ consisting of a series of nephrons that filter urine from both the pericardial cavity fluids via openings called nephrostomes and the bloodstream from the glomerulus. Cells of the nephron tubule may secrete nitrogenous wastes into the urine and reabsorb water and nutrients. Urine passes from the nephrons into one of two long tubes, the Wolffian ducts, which run along either side of the body cavity and empty into a bladderlike urogenital sinus.

In more advanced vertebrates the pronephros is the first kidney to develop in the embryo. Frequently nonfunctional, it is soon replaced (after 3 ¹/₂ weeks in humans) by the mesonephros, which lacks nephrostomes and draws fluid from the glomerulus only.