The present invention provides pharmaceutical compositions comprising droxidopa alone, or in combination with one or more further active ingredients, for the treatment of conditions, such as mood disorders, sleep disorders, or attention deficit disorders. In certain embodiments, the compositions useful in the methods of the invention comprise droxidopa and a compound selected from the group consisting of DOPA decarboxylase inhibiting compounds, catechol-O-methyltransferase inhibiting compounds, cholinesterase inhibiting compounds, monoamine oxidase inhibiting compounds, norepinephrine reuptake inhibiting compounds, selective serotonin reuptake inhibiting compounds, tricyclic antidepressant compounds, serotonin norepinephrine reuptake inhibiting compounds, norepinephrine dopamine reuptake inhibiting compound, noradrenergic and specific serotonergic antidepressants, and combinations thereof. The inventive compositions are particularly useful in the treatment of depression, narcolepsy, insomnia, and Attention Deficit/Hyperactivity Disorder (AD/HD).