Anterogradely and retrogradely transported membranous organelles were analysed separately by focally cooling axons (cold-blocking) for 2-4 h. Video-enhanced differential interference contrast light microscopy (AVEC-DIC) and dark field light microscopy showed that particles accumulated in large numbers on both the anterograde and the retrograde sides of the cold-block and that the accumulated particles resumed their transport when the preparation was rewarmed to 18 degrees C. The particles accumulated in files on both sides of the cold-block suggesting that particles move along linear pathways in the axoplasm. Comparisons of the results obtained by AVEC-DIC light microscopy with those obtained by electron microscopy indicate that the AVEC-DIC method is capable of detecting all of the different types of rapidly transported membranous organelles, including the smallest (35-80 nm) vesicles that move anterogradely. Electron microscopic analyses of the transported particles demonstrate that the anterogradely transported organelles are structurally distinct from those that are transported retrogradely. The anterogradely transported particles consisted of normal mitochondria and small (35-80 nm) tubulovesicular profiles. By contrast, the retrogradely transported particles were 150 nm or larger and they often contained complex membranous inclusions. The largest retrogradely transported particles appeared to be degenerating mitochondria. The results are consistent with the hypothesis that the direction of organelle movement is related to the physiological state of the organelle. That is, organelles containing newly synthesized membrane components move primarily anterogradely and organelles that contain transformed and degraded membrane components move retrogradely.