1. In cats anaesthetized with alpha-chloralose extracellular recordings were made from fine afferent units belonging to the medial articular nerve of the knee joint. The excitatory and sensitizing effects on articular afferents of prostaglandin E2 (PGE2) applied intra-arterially close to the joint were examined. 2. Bolus injections of PGE2 doses of 0.03-30 micrograms excited about 60% of both the group III (conduction velocity 2.5-20 m/s) and the group IV units (conduction velocity less than 2.5 m/s). The duration and size of the responses were dose dependent consisting in most cases of low-frequency discharges which lasted up to several minutes. Excitation was found among afferents with low and high mechanosensitivity. 3. Among the group III units PGE2 sensitized 64% for their responses to movements and 50% for their responses to bradykinin (applied intra-arterially close to the joint). Sensitization did not depend on the mechanical threshold previous to chemical stimulation. Among the group IV units PGE2 sensitized only 25% for their responses to movements but 75% for their reactions to bradykinin. In group IV fibres a low mechanical threshold predisposed for sensitization to movements and a higher threshold for sensitization to bradykinin. 4. Some units were sensitized and excited, others were either sensitized or excited and some units were not affected by PGE2. We conclude that PGE2 induces in a large proportion of fine articular afferents of normal joints discharges which are similar to those induced by an experimental inflammation. Thus PGE2 may be an inflammatory mediator which has a major role in the generation of the afferent activity developing in the course of an arthritis.