Conduction velocities, demyelination, "macrophage/dendritic" cells, different sets of T-lymphocytes, and immunoglobulins were estimated in sciatic nerves during various phases of experimental allergic neuritis in Lewis rats. Demyelination was minimal day 15 postimmunization (p.i.) when conduction velocity already was reduced, somewhat more pronounced day 17 p.i. when nerve conduction was blocked, and most pronounced day 23 p.i. when nerve conduction partially had recovered. This suggests a dissociation between the degree of demyelination and the functional deficits. Decrease of sciatic nerve conduction velocities coincided with endoneurial appearance of T-lymphocytes and "macrophage/dendritic" cells, as well as endoneurial immunoglobulins, day 15 p.i. Later partial functional recovery occurred in parallel with the disappearance of T-cells. The degree of functional deficits thus correlated with the number of endoneurial T-lymphocytes. T-cells may, directly or indirectly, initiate several of the disease components in experimental allergic neuritis, including the nerve conduction deficit.