Our previous observations have revealed that the total time spent in immobility and time to reach complete immobility (latency) vary with the diameter of the cylindric chamber where mice are forced to swim in the behavioral despair test. Therefore, we investigated the effect of changing the test conditions of the original Porsolt test. Mice were forced to swim for 15 min in chambers with 10 cm (original diameter of the Porsolt's forced swimming chamber), 20, 30, and 50 cm diameter in 20 cm deep water. Total time spent in spells of immobility during the observation period from third to sixth (inclusive) minutes and time to reach complete immobility were measured. In addition, a possible correlation between the rotatory locomotor activity of mice during swimming as assessed by the number of tours per minute and effect of antidepressant drugs on it was investigated. Total duration of spells of immobility was shorter and the latency was longer in tests carried out in chambers with 10 cm diameter. Increasing the diameter of the cylinders made it possible to distinguish the antidepressant drugs from caffeine, anticholinergics, and antihistaminics, which gave a false positive response in 10 cm diameter cylinders, but not in cylinders with larger diameters. Increasing the diameter of the chambers to 20 and 30 cm also allowed to study the selective effect of the antidepressants, namely, the rotatory locomotor activity during swimming. The extension of the test period to 15 min increased the reliability of the measurement of the time to reach complete immobility. These results indicate that the modifications described in this work increase the predictive power of the original Porsolt test used in screening the chemicals with a potential antidepressant property. © 1994.