Spontaneous and drug-induced (haloperidol, apomorphine, and amphetamine) motor activity of rats was measured simultaneously via two distinct and independent methods : the classical optical scanning technique and a new procedure based on the piezo-electric principle. The latter procedure measured animal-induced mechanical vibrations of a flexible cage floor which were transduced into electric signals via piezo-electricity. The piezo method appeared to be relatively more sensitive in recording the small, stereotyped motor movements induced by apomorpine (0.63->10mg/kg) and high doses of amphetamine (2.5-__> 20 mg/kg). The optical scanning technique, on the other hand, was more sensitive in recording horizontal displacements across the cage such as induced by low doses of amphetamine (0.31-2.5 mg/kg). Both methods showed comparable sensitivity in recording the depression of behaviour induced by haloperidol (0.04->1.25 mg/kg) or low doses of apomorphine (0.04-0.16 mg/kg), The piezo method may complement the optical scanning procedure, and thereby enhance the information on the extent that test compounds modify animal behaviour.