SUMMARY Introduction: To assess an effect of cochleostomy on hearing threshold in guinea pigs. Material and methods: The authors performed animal experiments using fi ve 3-month-old guinea pigs. Before experiment hearing threshold were evaluated. Surgery involved access to the temporal bone by a post-auricular incision. After a wide opening of the bulla cochleostomy was created (10 000 turn/min, diamond bur of 0,8 mm diameter). Hearing threshold was identifi ed on the basis of presence of wave V in auditory brainstem responses (ABR) for click and frequency-specifi c stimulation. Also morphology and latency changes for wave V for this stimulation was assesed. Hearing status was evaluated before, just after and 1-, 2-, and 4-weeks after surgery. For surgical procedure and ABR examination all animals were anesthetized with an intramuscular injection of ketamine (50 mg/kg) mixed with xylazine (9 mg/kg) in the supplemental doses. After surgery the animal was treated by antibioticoterapy for 3 days – Enrofl oksacyna 0,3 ml subcutaneouly and analgesic – Tolfedine 0,05 mg in second day. Results: Four week observation of ABR morphology and hearing thresholds for click and frequency-specifi c stimulation of 100 dB SPL intensity showed only temporary changes confi rming that cochleostomy did not affect cochlear function. Conclusions: The correctly performed cochleostomy in guinea pigs did not affect persistently the cochlear function indicating that such an option of CI electrode insertion in patients is safe.