The spatial relationship between microtubules and mitochondria was studied in myelinated axons of the ventral and dorsal spinal roots of the lizard Lacerta muralis by use of quantitative methods in single and serial sections. Microtubules mainly occurred in groups of 3 to 10. The mean density of microtubules was found to be significantly higher close to mitochondria than in the rest of the axoplasm. In single sections, 59-62% (according to the root region examined) of the microtubule groups were found to be 'associated' with mitochondria; this percentage rose to 74-76% in serial sections. The examination in serial sections of progressively longer segments of the same microtubule groups showed that the longer the segments of microtubule groups examined the higher was the percentage of microtubule groups 'associated' with mitochondria. The results obtained show that in the axons studied in the present research a non-accidental spatial association exists between microtubule groups and mitochondria. This evidence supports the suggestion that the microtubule groups play a role in the movement of mitochondria along the axon, even though it does not clarify the precise nature of this role.