[3H]acetylcholine (ACh) or [3H]gamma-aminobutyric acid (GABA) was injected into the cell body of a cholinergic neurone of Aplysia kurodai. 2. [3H]ACh moved down the axon at a maximum speed of 2.5 mm/sec at 20 degrees C. 3. 20 mM-colchicine suppressed this movement, but some passive movement of radioactivity was noted along the axon. 4. Profiles of the passive movement coincided with theoretically obtained diffusion profiles. 5. The diffusion coefficient of ACh in the axoplasm was estimated. It was 3 x 10(-6) cm2/sec at 5 degrees C, 4 X 10(-6) cm2/sec at 15 degrees C and 6.5 x 10(-6) cm2/sec at 30 degrees C. The Q10 was 1.35, the activation energy was about 5 kcal/degrees C. These diffusion coefficients were lower than that of free diffusion of ACh (8 x 10(-6) cm2/sec at 18 degrees C, Fatt, 1954), and assumed to be reasonable, if one considers frictional resistivity of axoplasm in the diffusion of ACh molecules. 6. [3H]GABA diffused similarly to ACh, and the diffusion coefficients agreed with the estimated value when the molecular size differences were taken into account. 7. Both ACh and GABA seemed to diffuse in soluble form or as single molecules in the axoplasm. 8. Intra-axonal diffusion is very effective for short distances.
