This study investigated in vitro effects of cannabinoids on the activity of monoamine oxidase, the enzyme responsible for metabolism of monoamine neurotransmitters and affecting brain development and function. The effects of the phytocannabinoid Δ9-tetrahydrocannabinol, the endocannabinoid anandamide, and the synthetic cannabinoid receptor agonist WIN 55,212-2 on the activity of monoamine oxidase were measured in a crude mitochondrial fraction isolated from brain cortex.