In a double-blind trial in 20 human volunteers, the duration of intradermal anesthesia with a mixture of bupivacaine, 0.5 mg, and Chloroprocaine, 3 mg, was compared with the duration of intradermal anesthesia obtained with either bupivacaine, 0.5 mg, or Chloroprocaine, 3 mg, alone. The duration of intradermal anesthesia produced by the bupivacaine mixed with Chloroprocaine, 36 ± 16 (mean ± 1 SD) minutes, was significantly less than the duration of the same amount of bupivacaine alone, 139 ± 78 minutes. Duration of anesthesia with Chloroprocaine alone, 24 ± 11 minutes, was similar to the duration of anesthesia with Chloroprocaine plus bupivacaine. A possible explanation for these results is that bupivacaine competes with Chloroprocaine for nonspecific receptors at the biophase and in the surrounding nerve tissue.