Effects of intermolecular hydrogen-bonding interactions on the amide I mode of N-methylacetamide (NMA) are studied by matrix-isolation infrared (IR) spectroscopy and ab initio molecular orbital calculations. The wavenumbers of the amide I IR bands of NMA in Ar and Nz matrixes with various NMA/matrix gas mixing ratios are compared with those calculated for the monomer, dimers, and trimers of trans-NMA and the monomer and dimer of cis-NMA. The band at 1708 (1706) cm(-1) in Ar (N-2) matrixes is assigned to the amide I mode of the monomer of trans-NMA. The band observed at 1686 (1681) cm(-1) in Ar (Nz) matrixes with the NMA/matrix gas mixing ratio larger than 1/500 is assigned to the amide I band due to the dimers of trans-NMA. The bands observed at lower wavenumbers for samples with the NMA/matrix gas mixing ratio as large as 1/100 are assigned to the amide I bands of the trimers and larger clusters of trans-NMA. It is likely that the band observed at 1695 (1693) cm(-1) in the Ar (N-2) matrix arises from the dimer of cis-NMA, which is as stable as the trans-NMA dimers because of the formation of two hydrogen bonds in a cyclic form. Although there are two amide I modes in an NMA dimer and three in an NMA trimer, only one mode is strongly IR active in each species. The intrinsic amide I wavenumbers of individual peptide groups in NMA clusters, i.e., the amide I wavenumbers in the case where there is no resonant vibrational coupling between the peptide groups, are examined by calculating the amide I wavenumbers for the dimers and trimers whose constituent molecules other than the target molecule have the C=O group(s) substituted with C-13 and O-18. It is shown that the shifts of the amide I band to lower wavenumbers induced by hydrogen bonding to the C=O group are 20-25 cm(-1), while those induced by hydrogen bonding to the N-H group are 15-20 cm(-1). These shifts are approximately in line with the changes in the C=O bond lengths and are approximately additive if both the C=O and N-H groups of a peptide group are hydrogen bonded.