3-N-[11C]methylspiperone(11C-NMS, molecular weight = 409 for 12C-NMS) has been prepared automatically with high specific activity (37 +/- 18 GBq/mumol at EOS) for the measurement of dopamine D2 receptors in the human brain with a positron camera. It was observed that the radiochemical purity of 11C-NMS decreased from 97.8% (in HPLC eluate) to 83.5% (in saline, after the dispensing procedure, at 29 min from EOS), and to 79.9% in a further 59 min and that another 11C-labelled compound with a molecular weight of 425 (as a 12C-labelled compound) was formed. It was also observed that an aqueous solution of carrier methylspiperone (NMS) decomposed on 60Co irradiation and, using the same analytical conditions, gave the compound with the same retention time and the same molecular weight as 11C-NMS gave. The G-value of NMS decomposition by 60Co irradiation was estimated to be about two in an aqueous solution. The decomposition of 11C-NMS was suppressed remarkably by the addition of hydroxyl radical scavengers such as potassium iodide, and accelerated slightly by a hydrated electron scavenger such as sodium nitrate. Therefore, it was assumed that the hydroxyl radical generated by the radiolysis of water played an important role in the decomposition of 11C-NMS in the aqueous solution. Polysolvate-80 and ethyl alcohol were used as i.v. injectable additives to protect 11C-NMS against decomposition. In 47 routine productions, 2.7 +/- 1.7 GBq of 11C-NMS aqueous solutions ready for i.v. injection have been produced at 98.3 +/- 1.0% radiochemical purity using an automated synthesis apparatus. The products were used for clinical purposes with a positron camera and animal experiments.