Investigations of the anatomy of the vestibular apparatus of the budgie, pigeon, and red-tailed hawk revealed evidence that all the gelatinous vestibular structures (cupulae, otolithic membranes, tectorial membranes) have contact not only with surfaces of hair cells but also with additional surfaces not covered by hair cells. The inner ears of birds are large even when compared to mammals of body weight 100 times greater, and birds are, therefore, particularly suitable models for investigations of such phenomena as alcohol nystagmus, heavy water nystagmus, and inner ear damage caused by deep diving. As expected, pigeons were found to have a vigorous positional alcohol nystagmus and also a vigorous heavy water nystagmus. In one pigeon, administration of oxygen-18 water was followed by a positional nystagmus of the heavy water type, and this result is taken as evidence for the view that heavy water nystagmus is caused by a density effect of the heavy water molecule and not by an effect of the deuterium atom on central enzyme systems.