The acute effects of alcohol on spontaneous locomotor activity in male Swiss mice were studied at various times after an IP injection of 2 g/kg ethanol. Subjects were placed alone in a novel arena and videotape recordings were made of behaviour: trials were of 500-s duration and commenced at either 30, 60, 120, or 180 min after alcohol administration. Measures of behaviour included various indices of ambulation and immobility, together with a more detailed ethological analysis of the frequencies of all other acts and postures shown by test animals. Ambulation showed a biphasic response to alcohol treatment, consisting of an initial stimulation followed by a suppression after 3 h. Immobility was also increased by alcohol, and showed peak stimulation in trials commencing 30 min after administration: thereafter there was a progressive return to baseline levels. Many behavioural elements were suppressed including rearing, digging, shaking, and abbreviated grooming. Ethanol thus appeared to produce two distinct types of depression, in terms of increased immobility (and suppression of other behaviour) and in terms of decreased ambulation, the latter occurring when immobility had returned to baseline levels.