The primary sensory innervation of muscle spindles obtained by muscle biopsy of normal human volunteers was studied with the light and electron microscopes. The parent IA sensory fibre branched 4-6 times, became unmyelinated for 25-30 μm, then formed sensory terminals on each nuclear bag and chain intrafusal muscle fibre. The first 4-5 μm of the unmyelinated segment is believed to be an encoder zone because the plasmalemma was undercoated by a dense granular layer similar to that under other membranes where action potentials originate. A reconstruction from micrographs of serial longitudinal sections showed that the primary sensory ending on a nuclear bag fibre is an irregular coil with branches and varicose swellings. The terminals contain central aggregates of microfilaments often surrounded by mitochondria, small numbers of vesicles, cisterns and tubular profiles. The latter merge with the plasma membrane. Junctional complexes between the plasma membranes of the terminals and intrafusal muscle resemble fascia adherns and are postulated to act as attachment plaques. These could contribute to the transduction process by increasing the degree of distortion of the terminal's membrane when stretch is applied to the spindle. A mechanism is described which could account for some of the differences in sensitivity of the primary and secondary sensory endings.