The intradental nerve activity was recorded from single pulp nerve unit dissected from the inferior alveolar nerve in canine teeth of anesthetized cats. The effects of various test solutions on intradental nerve activity evoked by 4M NaCl applied to the deep dentinal cavities were investigated. 1. Total 15 single pulp nerve units were recorded. Of these 9 were Mi - fibers and 6 were C -fibers. The mean conduction velocity and electrical threshold of $A{delta}$ - fiber were $6.3{pm}3.7m/sec$, $1.2{pm}0.7V$ and those of C - fiber were $1.0{pm}0.2m/sec$, $2.3{pm}1.3V$, respectively. The response to cold stimuli of $A{delta}$ - fiber began immediately and that of C - fiber began after a latency. 2. When applied to deep dentinal cavity, 4M NaCl induced irregular bursts of action potential in 4 $A{delta}$ - fibers and 3 C - fibers, which continued until the solution was washed away. 3. In the $A{delta}$ - fiber, histamine failed to induce any nerve acitivity and did not produce an increase in intradental nerve activity evoked by 4M NaCl. However following the application of 1M KCl, the response to 4M NaCl was eliminated. 4. In the C - fiber, histamine generated some nerve activity and produced a significant increase in intradental nerve activity evoked by 4M NaCl, but 5M $CaCl_2$ did not abolish this enhandced response.