Aaptamine is a marine sponge alkaloid originally isolated from A. aaptos with diverse biological activities, including antiproliferative and antidepressant properties. It is an agonist of δ- and μ-opioid receptors (EC50s = 5.1 and 10.1 μM, respectively, in HEK293 cells expressing human recombinant receptors) and a competitive antagonist of α-adrenergic receptors (α-ARs; pA2s = 4.88 and 5.43, respectively, in isolated rabbit aorta and renal artery). Aaptamine inhibits growth of HeLa cervical cancer (IC50 = 15 μg/ml) and K562 leukemia (GI50 = 10 μM) cells and induces cell cycle arrest in the G2/M phase in K562 leukemia and MG63 osteosarcoma cells. It is a proteasome inhibitor that inhibits chymotrypsin-, caspase-, and trypsin-like activity in a partially purified rat liver proteasome preparation (IC50s = 1.6, 2.7, and 18 μg/ml, respectively) and dose-dependently activates p21 independent of p53 in MG63 cells when used at concentrations ranging from 20 to 50 μM. Aaptamine (40 mg/kg, i.p.) decreases immobility time in the forced swim test in mice, indicating antidepressant-like activity.