A controlled, quantitative investigation of the electroencephalogram (EEG) and transcendental meditation (TM) revealed that EEG changes during TM were rarely as pronounced or consistent as previous reports suggest. There was considerable variation between subjects, some displaying no EEG changes at all during TM compared with an equal period of non-meditation. Any changes that did occur in a particular individual were not necessarily repeated in a subsequent session. A comparison of mean EEG parameters of the experimental group revealed no consistent significant differences between meditation and non-meditation, although trends towards increased theta and decreased beta activity during meditation were apparent. The biggest differences in mean EEG parameters were between subject groups. In particular, the group of meditators exhibited significantly more theta activity (during both TM and non-meditation) than a randomly selected group of individuals that had never meditated or been hypnotized. The EEG characteristics of the group of meditators were similar to those of a group of subjects experienced in self-hypnosis. It is concluded that the most obvious EEG changes during meditation are long-term. In people who regularly practise TM (or self-hypnosis), the EEG gradually (over weeks or months) tends to "slow down." Such a "slowed down" EEG is apparent during both normal waking conditions and altered states of consciousness in these individuals.