The G-protein linked receptor, proteinase-activated receptor-4 (PAR₄) is activated by proteinases released into the joint during inflammation. It is unclear whether PAR₄ has a pro- or anti-nociceptive effect and whether it directly affects nerve activity. In this study we examined the expression of PAR₄ in joints and DRG neurons and whether activation of PAR₄ has an effect on nociception in normal rat knee joints. Electrophysiological recordings were made from joint primary afferents in male Wistar rats during both non-noxious and noxious rotations of the knee. Afferent firing rate was recorded for 15 minutes post close intra-arterial injection of 10^-9 - 10^-5 mol of the PAR₄ activating peptide, AYPGKF-NH₂, or the inactive peptide, YAPGKF-NH₂ (100μl bolus). Rats were either naive or pre-treated with the selective PAR₄ antagonist, pepducin P4pal-10, the transient receptor potential vanilloid-1 (TRPV1) antagonist, SB366791 or the bradykinin B₂ receptor antagonist, HOE140. Immunofluorescence experiments showed extensive PAR₄ expression in the knee joint and in sensory neurons projecting from the joint. AYPGKF-NH₂ significantly increased joint afferent firing during both non-noxious and noxious rotation of the knee. The inactive control peptide, YAPGKF-NH₂ was without effect. Systemic pre-treatment with the PAR₄ antagonist, pepducin P4pal-10, inhibited the AYPGKF-NH₂-induced increase in firing rate. Pre-treatment with HOE140, but not SB366791, also blocked this increase in firing rate. This data reveal that in normal rat knee joints, PAR₄ activation increases joint primary afferent activity in response to mechanical stimuli. This PAR₄-induced sensitisation is TRPV1-independent but involves B₂ receptor activation, suggesting a role for kinins in this process.