The methylguaniDine derivative 11C-GMOM (11C-labeled N-(2-chloro-3-thiomethylphenyl)-N9-(3-methoxyphenyl)-N9-methylguaniDine) has been used successfully to quantify N-methyl-D-aspartate (NMDA) receptor binDing in humans. The purpose of the present study was to estimate the 11C-GMOM raDiation dose in healthy humans. Methods: After 11C-GMOM injection, 3 female and 2 male subjects underwent 10 consecutive whole-body PET scans in approximately 77 min. Seven source organs were defined manually, scaled to a sex-specific reference, and residence times were calculated for input into OLINDA/EXM software. Accepted tissue-weighting factors were used to calculate the effective dose. Results: The mean absorbed raDiation doses in source organs ranged from 7.7 mGy-MBq21 in the brain to 12.7 mGy-MBq21 in the spleen. The effective dose (6SD) was 4.5 6 0.5 mSv-MBq21. Conclusion: The effective dose of 11C-GMOM is at the lower end of the range seen for other 11C-labeled ligands, allowing for serial PET scanning in a single subject.