In this study, we have measured the passive, the intrinsic, and the reflex-mediated mechanical response to stretch of the ankle extensors and flexors in 13 spastic multiple sclerosis patients and 10 healthy control subjects. In the ankle flexors, the patients had no reflex-mediated stiffness. The passive stiffness was increased by 138% (95% confidence interval: 26-91%) and the intrinsic stiffness by 79% (41-158%) when compared with the healthy subjects. In the ankle extensors, the reflex-mediated stiffness and the intrinsic stiffness of the patients were equal to the reflex-mediated and the intrinsic stiffness in healthy subjects. The passive stiffness was increased by 152% (41-352%). We conclude that spastic muscles in multiple sclerosis patients have an increased non-reflex stiffness (passive plus intrinsic stiffness), and that the reflex-mediated stiffness in the extensors during a sustained voluntary contraction does not differ significantly from healthy subjects.