Skilled manual responses depend upon information about the position of the hand which is to be moved. In order to throw light on the way in which CNS depressant drugs impair skill, an experiment was performed to study the effect of nitrous oxide on the perception of hand position by vision and by proprioception. The results show that these two modalities were less efficient in combination than was either separately. The drug increased the variability of performance irrespective of the perceptual conditions. The drug also produced systematic changes in constant error, by its action either on vision or on proprioception. Drug-induced increases in the size of handwriting may be explained as changes which compensate for the effects of the drug on perception.