Thirty-five cadavers were studied to determine the depth of penetration of the needle from the oral mucous membrane to the inferior alveolar nerve at the mandibular foramen. The depth of penetration at the narrowest anteroposterior width of the ramus and pterygotemporal depression was determined to be half the narrowest anteroposterior width of the ramus plus or minus 1 mm. This depth was within the limits of a short needle. It was easier to estimate the depth of penetration with a short needle than with a long needle. A short needle also was less likely to go too deep and deviate from its course than was a long needle.