The common spiny mouseAcomys cahirinus and the golden spiny mouseA. russatus coexist in the extreme arid and hot parts of the Arava Rift Valley in Israel. The coexistence of these two species is through exclusion ofA. russatus from nocturnal activity byA. cahirinus, which is nocturnal. An attempt was made to study the daily activity rhythm response ofA. russatus to chemical signals released byA. cahirinus, in order to understand the mechanism of segregation. For this purpose oxygen consumption,[Formula: see text]O2, was studied in individuals ofA. russatus kept alone in a metabolic chamber in a constant ambient temperature of 32°C under two photoperiod regimes: 12L:12D, and constant darkness (D:D). After three days of[Formula: see text]O2 recording under these conditions, chemical signals released fromA. cahirinus were passed through the metabolic chamber. This treatment caused a shift in the activity rhythm only ofA. russatus kept at 12:12D, while a decrease in[Formula: see text]O2 rates ofA. russatus was noted in both groups. These results suggest that chemical signals released byA. cahirinus may play a role in shiftingA. russatus toward its diurnal activity.