Dantrolene (1) is a substrate for breast cancer resistant protein, which is widely distributed in the blood–brain-barrier, intestine, gall bladder, and liver. PET study with 1 labeled with a positron emitter can be used to visualize BCRP and to elucidate the effect of BCRP on the pharmacokinetics of drugs. The objective of this study was to label 1 using nitrogen-13 ( 13 N, a positron emitter; half-life: 9.9min). Using no-carrier-added [ 13 N]NH 3 as the labeling agent, we synthesized [ 13 N]dantrolene ([ 13 N]1) for the first time. The reaction of carbomyl chloride 2b with [ 13 N]NH 3 gave an unsymmetrical urea [ 13 N]3, followed by cyclization of [ 13 N]3 to afford [ 13 N]1. Due to its instability, 2b was prepared in situ by treating amine 5 with triphosgene in a ratio of 4 to 1 and used for subsequent [ 13 N]ammonolysis without purification.