The open-field apparatus has been used to study withdrawal reactions from chronic treatments with central nervous system depressant drugs. To study the behavior of the same animal after drug withdrawal, the rats are introduced into the open field on consecutive days. Because the open field is a novel environment, the repetition could lead to false-negative results with regard to drug withdrawal. To overcome this problem, we sought a modification of the open field, using different floor-painting patterns every time the animal is observed. The most frequently observed withdrawal manifestation was hyperactivity. We verified that long-term treatment withdrawal reactions from barbital, clonazepam, and ethanol were seen more often if the rats were introduced in the modified open field. In addition, fewer animals were used here than in other trials and hyperactivity was detected more frequently in the modified open field than was sound-induced convulsions. We propose that the modified open field is more useful than the classic one for screening of drug withdrawal reactions.