Allopregnanolone (3a-hydroxy-5a-pregnan-20-one or 3a,5a-tetrahydroprogesterone), generally abbreviated as ALLO or as 3a,5a-THP, is an endogenous inhibitory pregnane neurosteroid. It is synthesized from progesterone, and is a potent positive allosteric modulator of the action of gamma-amininobutyric acid (GABA) at GABA_A receptor. Allopregnanolone has effects similar to those of other positive allosteric modulators of the GABA action at GABA_A receptor such as the benzodiazepines, including anxiolytic, sedative, and anticonvulsant activity.