We present the tip and friction forces acting on a needle during penetration into a canine prostate, independently measured by a 7-axis load cell newly developed for this purpose. This experimental apparatus clarifies the mechanics of needle penetration, potentially improving the development of surgical simulations. The behavior of both tip and friction forces can be used to determine the mechanical characteristics of the prostate tissue upon penetration, and the detection of the surface puncture, which appears in the friction force, makes it possible to estimate the true insertion depth of the needle in the tissue. The friction model caused by the clamping force on the needle can also be determined from the measured friction forces.