The1H,199Hg and13C spectra of the more common of the isotopic species occurring in natural abundance in mercury dimethyl have been studied. The proton spectra were observed directly whilst the199Hg and13C spectra were measured indirectly by a double resonance method involving observation in the proton region. A hitherto unreported long range coupling of magnitude 0·44±0·02 c/s was observed between the methyl protons and new values were obtained for the directly bonded13C-H coupling, ± 129·6±0·1 c/s, and for the geminal199Hg-12C-H coupling, ± 101·9±0·1 c/s. The relative signs of these couplings, together with the sign and magnitude of the directly bonded199Hg-13C coupling, ±689±2 c/s, were determined by experiments on the 0·33 per cent abundant species H313C199Hg-12CH3. The magnitude of the199Hg-13C coupling is discussed in terms of the correlation between reduced coupling constants and atomic number. The vicinal13C-Hg-12CH3coupling appears to be zero.