The aqueous extract of Hydrilla verticillata (AEHV) was tested for possible pharmacological effects on experimental animals. AEHV significantly potentiated the sleeping time of mice induced by standard hypnotics viz. pentobarbitone sodium, diazepam, and meprobamate in a dose dependent manner. AEHV showed significant analgesic properties as evidenced by the significant reduction in the number of writhes and stretches induced in mice by 1.2% acetic acid solution. It also potentiated analgesia induced by morphine and pethidine in mice. Pretreatment with AEHV caused significant protection against strychnine and leptazol-induced convulsions. The behavioral studies on mice indicate CNS depressant activity of the aqueous extract of H. verticillata.