1. Axonally transported organelles were detected optically in myelinated axons from Xenopus laevis at room temperature (21-23 degrees C). Details of the motion of organelles which were transported in the retrograde and anterograde directions were studied using filmed records.2. A group of 133 organelles with a mean retrograde velocity of 0.91 mum/sec was compared with a group of thirty-nine organelles with a mean anterograde velocity of 0.93 mum/sec.3. Averaged power spectra of the positional deviations about the mean positional change through time were constructed for organelles which travelled in the retrograde and anterograde directions. Most of the power in the two spectra was at frequencies below 0.2 Hz and each contained a single peak at 0.02-0.04 Hz. The power spectrum for retrograde organelle motion had a magnitude about twice that for anterograde organelle motion.4. Estimates of the instantaneous velocity of organelles which travelled in either direction varied smoothly with time. Instantaneous velocity was not a smooth function of organelle position, (i.e. was ;saltatory').5. Histograms of the estimates for the groups of organelles whose major motion was retrograde or anterograde were broad, covering a range of about 3 mum/sec, were unimodal, and passed through zero to include a small group of values which indicated motion in the opposite (minor) direction.6. Organelles spent, on average, more time moving in the minor direction the lower their mean velocity.7. The variation in instantaneous velocity was greater for organelles which travelled in the retrograde direction than for those which travelled in the anterograde direction. No correlation was found between the variation of instantaneous velocity and the mean velocity of the organelles.8. Images of organelles occasionally appeared to rotate while the organelle continued to move in the major direction of travel.9. Evidence is presented that spatially related properties of the axon influence organelle velocity and that this influence is common to organelles which travel in the two major directions.10. A hypothesis is presented to account for the findings. This supposes that each organelle travels through a stationary axoplasm and is propelled by the resultant of two opposing driving forces whose relative magnitude fluctuates with time. Spatially dependent properties of the axoplasm modify the postulated time-related cycle of motion.