1.　The　purpose　of　the　present　study　was　to　investigate　whether　there　was　a　cooperative　interaction　between　substance　P　(SP)　and　glutamate　(GLU)　administered　subcutaneously　on　A　delta　and　C　primary　afferent　fibre　activity　in　dorsal　hairy　skin　of　the　rat　in　vivo.　The　single　unit　activities　of　A　delta　and　C　afferent　fibres　were　recorded　by　isolation　of　fibre　filaments　from　the　dorsal　cutaneous　nerve　branches　and　the　effects　of　subcutaneous　injections　of　low　doses　of　SP,　GLU　and　SP　+　GLU　on　activity　were　determined.　2.　Sub-threshold　doses　of　SP　(1　mu　mol/L,　10　mu　L)　administered　subcutaneously　into　the　dorsal　hairy　skin　had　no　effect　on　the　afferent　discharges　of　either　A　delta　or　C　units.　3.　The　afferent　discharges　of　35%　(11/31)　of　A　delta　fibres　and　33%　(6/18)　of　C　fibres　were　increased　by　local　injection　of　the　submaximal　doses　of　GLU　(10　mu　mol/L,　10　mu　L)　into　the　receptive　fields.　4.　The　GLU-induced　excitatory　response　was　significantly　enhanced　by　coinjection　of　subthreshold　doses　of　SP.　The　mean　discharge　rates　of　A　delta　fibres　and　C　fibres　were　increased　from　5.84　+/-　1.54　and　5.02　+/-　2.65　impulses/min　to　19.91　+/-　4.35　and　17.58　+/-　5.59　impulses/min,　respectively,　whereas　the　excitatory　proportions　of　A　delta　and　C　fibres　were　increased　from　35　and　33%　to　84　and　83%,　respectively.　The　duration　of　the　excitation　for　A　delta　fibres　and　C　fibres　was　also　significantly　increased　after　coinjection　of　SP　+　GLU　compared　with　that　observed　when　either　substance　was　given　alone.　5.　The　present　study　provides　electrophysiological　evidence　for　an　interaction　between　receptors　for　SP　and　GLU　on　the　fine　fibres　activities　in　rat　hairy　skin,　which　may　be　involved　in　the　mechanisms　of　hyperalgesia.