In tile order to examine activity of the dopamine system in the Varitint-waddler (Va) mouse, behavioral effects of haloperidol and dexamphetamine on waltzing Va, non-waltzing Va and non-mutant mice were investigated. The dopamine system of these mice was studied by macro-autoradiography with 14Cdihydroxyphenylalanine. High doses (5.0-10.0 tzmol/kg) of haloperidol were similarly effective in both mutant and non-mutant mice: they induced a highly characteristic cataleptic effect in all types of mice. Low doses (0.10--3.16 [zmol/kg) were significantly less effective in the waltzing Va mice: they induced a marked shift from a small increase of the sitting frequency in the waltzing mice to a large increase of this measure in the non-mutant mice. Several behavioral measures of the untreated waltzing mice mimicked those of control mice treated with amphetamine. Amphetamine reversed original strain differences and depressed several stereotyped Va activities. The autoradiograms of the waltzing Va mice, which showed a marked labelling of the Harderian gland, differed significantly from those of the nonwaltzing Va mice and controls, which showed no labelling of this gland. It is suggested that a hyperactive dopaminergic mechanism is involved in the waltzingshaker syndrome of the Varitint-waddlcr.