Synthesis of deuterium‐labeled CB1 receptor antagonist 2‐d9 was accomplished in three steps by alkylation of 2‐nitrophenylacetonitrile with cyclopentyl‐d9 bromide, reductive cyclization of the resulting secondary nitrile into the 3‐cyclopentyl indole‐d9 and its N‐sulfonylation with corresponding p‐amidosulfonyl chloride. Another, structurally related, CB1 receptor antagonist 1 was radiolabeled with carbon‐14 by oxidative cleavage of 3‐cyclopentyl indole followed by the ring closure of o‐acyl substituted N‐formylaniline with potassium cyanide‐14C, in situ reduction‐elimination of the intermediate amino alcohol, and N‐sulfonylation of the resulting 3‐cyclopentyl indole‐2‐14C. Synthesis of deuterium‐labeled CB1 receptor antagonist was accomplished in three steps from cyclopentyl‐d9 bromide, through the 3‐cyclopentyl indole‐d9 and its N‐sulfonylation. Another, structurally related, CB1 receptor antagonist was radiolabeled with carbon‐14 by the ring closure of o‐acyl substituted N‐formylaniline with potassium cyanide‐14C, in situ reduction‐elimination of the intermediate amino alcohol, and N‐sulfonylation of the resulting 3‐cyclopentyl‐2‐14C‐indole.