The hyperlocomotor effect of the serotonin (5-HT)1A,B receptor agonist 5-methoxy-3-(1,2,3,6-tetrahydro-4-pyridinyl)-1H-indole (RU 24969) has been repeatedly reported. However, 5-HT1A receptors, 5-HT1B receptors (or both) have been claimed to mediate this effect of RU 24969. These contradictory data possibly arise from protocol differences, especially those related to animal species, drugs, and activity assessment. Herein, the influence of a pretreatment with the selective 5-HT1B,D receptor antagonist N-[4-methoxy3-(4-methyl-1-piperazinyl)phenyl]-2'-methyl-4'-(5me thyl-1,2,4-oxadiozol-3-yl)-biphenyl-4-carboxamide (GR 127935; 1, 3.3 and 10 mg/kg IP) on the hyperlocomotor effect of a 5 mg/kg (IP) dose of RU 24969 was studied in Wistar-Kyoto Hyperactive (WKHA) rats. In a first series of experiments, it was confirmed that RU 24969 (2.5 and 5 mg/kg), administered 10 min after the onset of activity recordings, increases locomotion dose-dependently (cage crossings). In a second series of experiments, administration of GR 127935 10 min after the onset of activity recordings promoted a dose-dependent decrease in basal activity (and rearings) and prevented (3.3 and 10 mg/kg) RU 24969-elicited locomotor activity. On the other hand, GR 127935 was ineffective against RU 24969-induced inhibition of rearings. Lastly, it was observed that 3.3 mg/kg GR 127935 did not affect the number of cage crossings and rearings displayed by rats administered 1.5 mg/kg D-amphetamine. This study shows that 5-HT1B receptors play a major role in the hyperlocomotor effect of RU 24969, at least under our experimental setting. Whether these receptors also play a tonic role in the high locomotor activity displayed by WKHA rats remains to be determined.