6-Hydroxydopamine (6-OHDA) 10 and 40 micrograms/24 h infused bilaterally for 4 days into the rat substantia nigra (SN) caused a 'freezing' akinetic response which was apparent within 24 h and which persisted throughout the period of infusion. The infusion of 10 or 40 micrograms/24 h 6-OHDA into the SN failed to cause any change in motor coordination, or induce limb and body rigidity. The infusion of 40 micrograms/24 h 6-OHDA led to significant reductions in the striatal levels of dopamine, 3,4-dihydroxyphenylacetic acid, homovanillic acid and noradrenaline; no changes were observed following the 4-day infusion of 10 micrograms 6-OHDA. The consequences of infusing 6-OHDA into the SN are discussed in relation to those induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and 1-methyl-4-phenylpyridine (MPP+) where the spectrum of behavioural and biochemical change caused by 6-OHDA is shown not to mimic the actions of either MPTP or MPP+.