Ontogeny of immobility and response to alarm substance in the forced swim test.
The ontogeny of immobility in the forced swim test and the response to alarm substance were determined in rats. Immobility, defined as floating vertically in water, making only those minimal movements necessary to keep the head above water, emerged at 21 days of age and stabilized beginning at 26 days (< or = 10% variation from the mean time of the previous day). Males and females did not differ in maturation of this response. The reaction to alarm substance was discernible when animals were 24 days of age. Again, males and females were not significantly different in ontogeny of this response.