L-phenylalanyl-L-arginine (Phe-L-Arg) and L-phenylalanyl-D-arginine (Phe-D-Arg), dissolved in a physiological saline were injected intrathecally into the lateral brain ventricle, and their behavioural as well as analgesic effects in mice were determined. It was found that a 100 nM dose of either peptide intracerebroventricularly (i.c.v.) injected induced an increase in the locomotor activity and a convulsive effect, whereas the same dose if injected produced a significant analgesic effect. It has been concluded that Phe-L-Arg and Phe-D-Arg have significant but different central effects in mice.