An apparatus for studying the bone formation or bone ingrowth in an implant, in response to locally administered test substances of different types, is made of bio-compatible material, preferably pure titanium, and is implanted in living bone tissue, primarily for animal experiment activities. The apparatus has an outer portion with a central recess in which an inner portion is removably inserted. The two portions form, in the assembled state, a bone tissue ingrowth channel which is exposed when the inner portion is removed, whereby allowing examination of tissue which has grown into the channel. The inner portion includes a reservoir for the test substance whose bone formation promoting properties are to be tested. The reservoir is connected to the bone tissue ingrowth channel through a capillary aperture, so-called diffusion capillary, for supply of the test substance without simultaneous liquid flow.