The binding of 3H-spiroperidol to striatal membranes from a strain of mutant Han-Wistar rats was compared with that in normal littermate animals. The specific binding was less in the mutants than the controls. Scatchard analysis revealed that the KD- and Bmax-values for the high affinity binding sites in the mutants are greater than for those in the controls. These findings indicate that the dopamine receptors of the mutants are affected and could explain some of the previous data; it has been suggested that some of the spasticity observed in the mutants may be due to an abnormal functioning of their dopaminergic neurones.