Difference spectra between resting and excited nerve in the infrared region between 2000 and 1000 cm(-1) have been examined with a resolution of 0.5 cm(-1). Spectra were obtained with a modified Perkin-Elmer model 521 grating infrared spectrophotometer (Perkin-Elmer Corp., Instrument Div., Norwalk, Conn.), and the signal-to-noise (S/N) ratio was improved by time averaging and digital smoothing. Peaks occurring in the regions around 1030 and 1066 cm(-1) are identified as P-O-C stretch, at 1410-1414 cm(-1) as C-H deformation, and at 1750 cm(-1) as carbonyl stretch. The difference peaks appear to be due to a shift of about 1 cm(-1) in the absorption band to lower frequencies for the three lower frequency bands and to a higher frequency for 1750 cm(-1) band. Since the difference peaks appear when the nerve is modulated by a propagated action potential it is concluded that the changing electrical field across the nerve membrane is perturbing the absorption spectrum. From evidence presented it appears likely that these difference peaks are due to phospholipids in the nerve membrane and that they may be related to conformational changes associated with membrane permeability.