The cochlear blood flow was measured with the microsphere method in unanesthetized rabbits, prior to and during noise-exposure. Different sound pressure levels were used: 100 dB, 115 dB and 120 dB. Some of the animals had the cervical sympathetic trunk cut unilaterally. The effects of the delivered noise on the organ of Corti were studied in other rabbits with scanning electron microscopy. In spite of ultrastructural changes indicating damage of sensory cells after 115 dB and 120 dB exposure no changes of the total cochlear blood flow could be observed. After unilateral section of the cervical sympathetic trunk the cochlear blood flow was less on the intact side (25%) already prior to noise exposure and the difference was not significantly affected by noise. Different sizes of microspheres were used: 8.2, 8.4, 8.7, 9,2 mum. The 8.2 and 8.4 mum-spheres seemed well fitted for measurements of cochlear blood flow but passed the cerebral capillaries to a high extent. Control experiments using both 8.2/8.4 mum-spheres and 15 mum-spheres were performed and the degree of passage in different tissues was estimated.