6-(o-Chlorophenyl)-8-ethyl-1-methyl-4H-s-triazolo[3,4-c]thienol[2,3-e][1,4]diazepine (Y-7131), a new derivative of the thienodiazepines, had marked activities in antipentylenetetrazole effect in mice, attenuation of conflict behavior in rats, inhibition of aggressive behavior induced by hypothalamic stimulation in cats and muscle relaxant effects in normal and decerebrate cats. Y-7131 had weak activities in anti-MES effect in mice and loss of righting reflex in mice. The acute toxicity of Y-7131 was considerably lower than that of diazepam. No significant autonomic or neuroleptic effects were noted. Y-7131 appears to be a characteristic and potent anti-anxiety agent different from the benzodiazepines.