In order to discern early brain changes, fragments of hydrocarbons, 3-methyl-cholanthrene, 1,2,5,6-dibenzanthracene, 9,10-dimethyl-1,2-dibenzanthracene and azulene, were implanted cortically into rats and the brains examined 56 days later. Aside from 40%of the dimethylbenzanthracene-treated animals which presented inflammatory lesions, all displayed non-space-occupying lesions. The findings for dimethylbenzanthracene were similar when the intervening period was 128 days. BDF, mice injected intracerebrally with the above hydrocarbons as suspensions in saline and sacrificed 140 days later, displayed minimal brain changes.