In marmosets, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) causes degeneration of the cell bodies of the substantia nigra and the animals subsequently develop parkinsonian symptoms. Cystometrograms obtained from such animals while under pentobarbitone anaesthesia, showed their bladders to be hyperreflexic when compared to those of normal animals of the same age (less than 2 years). Bladder hyperreflexia is present in many parkinsonian patients and is difficult to treat, partly because it is made worse by dopaminergic agents. This is the first demonstration of an effect of MPTP on this type of peripheral function. It suggests the suitability of MPTP-treated marmosets for studying the mechanisms by which a loss of nigrostriatal dopamine leads to bladder hyperreflexia and for devising pharmacological strategies which may be of therapeutic value in the clinic.