The present study investigated the label distribution in several brain regions, as well as in the peripheral circulation, following injection of labelled tachykinins [( 3H]-substance P, [3H]-eledoisin or [125I]-neurokinin A) into the lateral cerebroventricle of the rat. A widespread label distribution, extending as far as to the brainstem, was detected. Hypothalamus, striatum and hippocampus were the most labelled regions by the 3 labels; however the patterns of distribution of the 3 labelled tachykinins showed marked differences. Distribution in the brain was rapid, reaching a maximum usually within 2 min after injection and declining slightly afterwards. Large amounts of label (1/6-1/15 of the total amount injected) were detected in serum even at 2 min after injection and increased thereafter, reaching a maximum at 10-15 min.