Endolymph is the fluid contained within the inner ear’s membranous labyrinth. It is sometimes also known as Scarpa's fluid, after Antonio Scarpa, the 18th century Italian anatomist who first discovered that the inner ear was filled with fluid. Endolymph has a composition very different from that of other extracellular fluids, with unusually high potassium levels. Even small changes in endolymph composition can harm hearing sensitivity.

In a process known as transduction, the inner ear converts sounds that enter the ear canal into the electrical signals that are carried to the brain. The inner ear contains a series of fluid-filled spaces. A structure called the bony labyrinth consists of a central vestibule, three semicircular canals and the spiral-shaped cochlea, and each substructure contains a corresponding portion of membranous labyrinth filled with endolymph. The utricle and saccule in the vestibule and the semicircular ducts in the semicircular canals are involved in maintaining balance, and the scala media in the cochlea is involved in hearing. Meanwhile, watery perilymph, a typical extracellular fluid comparable to plasma or cerebrospinal fluid, fills the space in the bony labyrinth surrounding the membranous membrane.

The endolymph is completely contained by the membranous labyrinth, with no connections to the perilymph. Endolymph has a unique high-potassium, low-sodium composition and a much higher total ion content than that of perilymph. The endolymph’s potassium is secreted from the stria vascularis. The high potassium content means that potassium, rather than sodium, maintains the electrical gradient in the cells here. Other significant features of endolymph are its low calcium content and high positive charge, known as the endocochlear potential.

The perilymph and endolymph do not flow between regions, as once was though. Rather, they are maintained by local ion transport processes. Unlike many other body fluids, endolymph and perilymph are not secreted and reabsorbed.

Meniere's disease is a disorder of the endolymph with symptoms including low-frequency hearing loss, ringing in the ears, dizziness and a feeling of pressure in the ear. In patients who have Meniere's disease, the amount of endolymph is dramatically increased, a condition known as endolymphatic hydrops. As a result, some part of the membrane surrounding the endolymph ruptures or develops a leak. The release of endolymph into perilymph is toxic and disturbs the normal chemical composition of the inner ear, leading to hearing loss. Also, rapid motion or spinning can cause movement of the endolymph, resulting in dizziness and possible motion sickness.