The advanced stage of Parkinson's disease (PD) is characterised by complex movement disturbances including freezing. Because freezing is resistant to drug therapy, there has recently been renewed interest in non-pharmacological treatment programmes. In this study the effect of rotational stimulation on freezing was investigated. Eight patients with idiopathic PD and freezing participated in the study. Switching from foot-lifting to 'stand up and walk' provoked freezing in all the tested patients. The mean OPMSP (Onset of Premotor Silence Period) in the EMG of the m. tibialis anterior during the 'foot-lifting' sessions preceding freezing was 124 msec. This value, together with a striking repetition of the EMG discharges in the m. tibialis anterior following the request to 'stand up and walk', was related to the inducement of freezing, as a decrease of 42 msec in the OPMSP (t = 2.61; p < or = 0.01) after rotational stimulation abolished freezing and the concomitant EMG disturbances. Rotational stimulation also reduced freezing frequency during daily life. Freezing periods fell to below 50% of the pre-treatment level the day following rotation (t = 5.58; p < or = 0.001). The OPMSP predicted the short and long-term effects of rotational exercises (R = 0.74; p < or = 0.03). The rather long-lasting effect of the stimulation suggests a possible modulation of neurochemical transmission. Further studies are required to shed more light on the point of action and to elucidate which neurotransmitter might be involved.