A concentration-related stimulation of anandamide (arachidonylethanolamide) synthesis by Δ 9 -tetrahydrocannabinol (THC) was observed in N-18TG2 neuroblastoma cells. Anandamide was detected and measured using an approach in which [ 3 H]arachidonic acid and [ 1 4 C]ethanolamine were incorporated into the phospholipids of subconfluent monolayers of cells, and the radiolabeled products were analyzed by TLC following agonist exposure. Both precursors showed similar concentration-response relationships and time dependencies consistent with the production of a product containing both the ethanolamine and arachidonyl moieties. The radiolabeled product also migrated together with authentic anandamide on two-dimensional TLC, confirming its identity as arachidonylethanolamide. Approximately two-thirds of the observed synthesis could be inhibited by 1 μM wortmannin, an agent previously reported to inhibit THC-stimulated arachidonic acid release. These findings are in agreement with reports showing that THC can mobilize phospholipid bound arachidonic acid, leading to the production of other eicosanoids.