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1. It was shown by Gandevia & McCloskey (1977) that anaesthesia of the thumb causes weights lifted by thumb flexion to feel heavier, and weights lifted by thumb extension to feel lighter. This was confirmed by Marsden, Rothwell & Traub (1979). Gandevia & McCloskey explained the effects in terms of altered motor command (and so, altered 'sense of muscular force, or heaviness') required when sources of peripheral reflex facilitation or inhibition were removed by anaesthesia. Marsden et al. proposed, instead, that co-contraction of antagonists altered in anaesthesia. They said that increases in heaviness occurred for flexion because anaesthesia was associated with increased activity in the thumb extensor and, similarly, that decreases in heaviness occurred for extension because anaesthesia reduced activity in the thumb flexor. 2. The apparent heaviness of weights lifted by flexion of the thumb is not systematically altered by paralysis of the extensor of the thumb (radial nerve block at the elbow). Heaviness is increased by local anaesthetization of the thumb (digital nerve block). If paralysis of the thumb extensor is induced after digital nerve block, no further alteration in apparent heaviness occurs. 3. If extensor co-contraction is encouraged for stabilization of the wrist, as by having subjects perform an isometric contraction with the index-finger while lifting weights by thumb flexion, the apparent heaviness of the weights is increased. If co-contraction of the extensors is prevented by radial nerve block the manoeuvre of simultaneous contraction of the index finger then fails to alter the heaviness of weights lifted by thumb flexion. 4. The middle finger can be postured in such a way that active extension but not active flexion of its distal joint is impossible. When co-contraction of the extensor cannot act at the distal joint, anaesthetization of the middle finger causes an increase in apparent heaviness of weights lifted by flexion of this joint. 5. The effects of digital anaesthesia on the apparent heaviness of objects lifted by digital flexion are not caused by co-contraction of the extensors of the digits, as proposed by Marsden et al. Where extensor co-contraction occurs unrelated to the digital flexion task, as in stabilization of the wrist during simultaneous flexion of more than one digit, it may affect the apparent heaviness of objects borne by individual digits.