Functional properties of lumbar mechanosensitive afferent fibres in the white rami L3 to L4 and the lumbar splanchnic nerves, which supply the retroperitoneal space were investigated. The receptive fields of these afferents, consisting of 1 to 3 mechanosensitive sites, were found on large vessels (aorta, inferior mesenteric artery, segmental arteries), on nerves, on the peritoneum, in fat lobules, on lymph nodes and on the vertebral column. Most afferent units were associated with small vessels. About 50% of the afferents had ongoing activity (about 0.1 to 2 imp/s). Two thirds of the afferents conducted at less than 2 m/s, the rest at 2 to 16 m/s. About 60% of the units were excited by local application of bradykinin and 75 to 90% by local application of hypertonic NaCl solution (4.5 and 9%) or KCl solution (60 and 155 mmol/l). About 50% of the afferent units with receptive fields on large arteries were activated by an increase in the arterial blood pressure. The function of these afferent fibres is unclear. However, it is conceivable that they are involved in nociception.