An endoscopic bite block, with teeth position defining regions which guide the subject's teeth to grip the bite block in their natural position, with the teeth of the upper jaw positioned further out than the lower jaw teeth. These teeth position defining regions are also preferably curved to match the curved shape of the jaw. The upper and lower parts of the front plate of the bite block may also be positioned at different distances out, so that upper and lower lip regions both contact the front plate snuggly. The bite block may be used with separate oral/nasal cannulae, or may incorporate breath sampling or gas supply cannulae. A flexible flapped curtain at the outer end of the bite block may be provided to largely enclose the inner volume of the bite block to enable more accurate capnographic sampling under conditions of wide-open mouth breathing.