Behavioural and biochemical effects of 2‐(2,6‐dichlorophenylamino)‐2‐imidazoline hydrochloride (St 155) on the central nervous system

St 155 (2-(2,6-dichlorophenylamino)-2-imidazoline hydrochloride) (0.1-2.5 mg/kg subcutaneously) in rats prolonged chloral hydrate sleeping time, inhibited exploratory activity, reduced rotarod performance and pain-induced aggression; in rats and guinea-pigs conditioned avoidance behaviour was inhibited.2. In most experiments St 155 was 5-7 times more potent than chlorpromazine on a weight basis. In white rats St 155 and chlorpromazine were equi-effective in inhibiting conditioned avoidance behaviour. St 155 was relatively less active than chlorpromazine in lowering body temperature.3. Chronic treatment with St 155 resulted in a reduction of its hypotensive and sedative effects and caused irritability and spontaneous episodes of severe fighting in white rats.4. St 155, at doses causing similar conditioned avoidance response inhibition, produced a significant increase in noradrenaline concentration in all brain regions except the striate. Radioisotope studies indicate an increase in noradrenaline storage without significant changes in metabolism.