The threshold shift at 500 Hz bone conduction under positive and negative static pressure of 400 mmH2O applied to the external auditory canal was examined in 20 patients of Meniere's disease with fluctuating low tone hearing loss, 21 patients of sensorineural hearing loss without fluctuating and 24 normal controls. In contrast to the 10 to 15 dB threshold shift in the latter two groups (non-fluctuating group, 14.8 +/- 5.1 dB at positive and 11.1 +/- 5.7 dB at negative; normal controls, 13.6 +/- 5.4 dB at positive and 9.3 +/- 5.3 dB at negative), the former showed a significantly smaller shift (3.2 +/- 6.1 dB at positive and 2.3 +/- 4.5 dB at negative) while hearing was depressed. However, when the hearing was improved, the pressure effect became greater even in the former group (11.3 +/- 5.3 dB at positive and 9.1 +/- 4.8 dB at negative). To understand further details of these phenomena, we performed electrophysiological animal experiments. Through a small hole made on the guinea pig's cochlear bony wall, 300 mmH2O pressure was applied to the scala tympani and the threshold of 500 Hz CM was examined in both endolymphatic sac obliterated ears and non-obliterated ears. In 14 non-obliterated ears 21.1 +/- 5.1 dB threshold shift was recorded, whereas it was only 1.6 +/- 2.6 dB in 9 obliterated ears at 4-6th postoperative days. These results seem to reflect that the endolymphatic pressure is great during the early stage of hydrops.(ABSTRACT TRUNCATED AT 250 WORDS)