Amino acid release in the dorsal horn of awake rats was examined by microdialysis during the development of arthritis induced by injection of 3% kaolin and 3% carrageenan into the knee joint. The following amino acids were measured by HPLC at baseline and for the first 8 h of arthritis: Asp, Glu, Asn, Gln, Ser, Gly and Tau. An initial increase in all amino acids examined was observed on injection of the knee joint with kaolin and carrageenan. Subsequently, there was a peak increase in Asp (184%), Glu (188%) and Gln (146%) during a prolonged release phase which began at 3.5 h and persisted at least 8 h. While Asn showed no changes from baseline, extracellular fluid concentrations of Ser, Gly and Tau were variable. This data indicates that the induction of arthritis is accompanied by an increased release of excitatory amino acids Asp and Glu which may be important in the generation of acute arthritis.