The authors investigated electroencephalographic (EEG) responses to tail clamping in lightly anaesthetized rat pups (5–22 days) in order to determine the ontogeny of EEG activity and at what age they may be capable of experiencing pain. Median frequency (F50) and spectral edge frequency (F95) of the power spectrum in the range of 1–30 Hz were determined before and after the application of a noxious stimulus and power spectra were compared by multivariate analysis. There was a postnatal increase in EEG power as, before clamping, pups aged 5–7 days exhibited isoelectric traces, whereas those aged 12–14 days and 21–22 days had intermittent EEG activity where the power in all frequencies was significantly lower at the former than at the latter age. Pups aged 5–7 days exhibited no EEG response to clamping in view of their isoelectric traces. Pups aged 12–14 days showed a significant decrease in F95 (P = 0.002), whereas those aged 21–22 days showed highly significant reduction in F50 and F95 (P = 0.028 and P < 0.001, respectively) as well as changes in EEG power of specific frequencies after clamping. The results and related literature suggest that rat pups aged 5–7 days and younger are not likely to perceive pain and that the ability to perceive pain develops gradually between postnatal ages 12–14 days and 21–22 days.