A potential threat to the long-term preservation of electrical and acoustical hearing in cochlear implant patients is the development of fibrous tissue and bone in the scala tympani. Simple impedance measures (impedance = voltage/current; Z = V/I), which are commonly used as a non-invasive test of implant integrity, might also be useful for monitoring the environment of the implanted cochlea. Increases in impedance might reflect growth of tissue that impedes current flow. Subsequently then in this study in guinea pigs, impedances for sinusoidal and pulsatile stimuli were measured with cochlear implants for up to 21 months and compared to histology at termination. Histology revealed variable distributions and amounts of scala tympani tissue near the primary measurement electrode. Impedances measured in the first 4 months after implantation increased and decreased over time, suggesting they were influenced by factors other than, or in addition to, tissue growth within the scala tympani. The magnitude of change in impedance for the sinusoidal stimuli was a moderate, statistically significant, predictor of the percentage of new bone and not fibrous or the total tissue in the long term. The extent of new bone was associated with long-term trends of increasing impedance. End-point impedances were highly correlated between bipolar and monopolar sinusoids, and monopolar sinusoids and pulses. Impedances for both stimuli and configurations showed that the amount of fibrosis was not predicted by impedance, but the presence of new bone was. The results should be helpful in determining the underlying causes of functional changes following cochlear implantation.