The C-18 hydroxy fatty acids ricinelaidic acid and ricinoleic acid diminish the oleic acid-stimulated agonist benzodiazepine binding in the rat brain in vitro. The oleic acid-induced enhancement of [3H]diazepam binding was completely abolished in membranes from the cerebellum, but only partially decreased in membranes from the hippocampus, cortex and the whole brain from 7-day-old rat pups. Related hydroxy fatty acids as well as hydroxy fatty acid esters had no effect on the oleic acid-stimulated diazepam receptor binding.