The effects of 3-acetylpyridine (3-AP) on the neurons in the inferior olive (IO) were studied by several methods to establish the time-order of events due to the neurotoxicity of 3-AP in the rat. It was found that IO metabolism, studied with [14C]2-deoxyglucose, began to decrease detectably 1 h after 3-AP and was totally suppressed at 3 h. Retrograde axonal transport of lectin horseradish peroxidase (HRP) from cerebellar cortex to the IO was also totally suppressed 3 h after 3-AP and in fact showed a time course similar to that for the suppression of metabolism. Harmaline produced tremor has been shown to induce rhythmic activity and increase glucose consumption in the IO. When injected in 3-AP treated animals, harmaline produced its usual effects at 2 h after the 3-AP but had no effects after 3 h. The present results indicate that the neurotoxic effects of 3-AP are not simply graded in time, but tend to have the greatest effects between the 2nd and 3rd hour following its administration.