The correlations between investigatory sniffing and rhythmic slow-wave activity (RSA) in the dorsal hippocampal formation were studied during free behavior in Fischer 344 rats aged 3, 18, 30, and 36 months. The amount and vigor of spontaneous exploratory behavior was reduced in older animals, and the frequency distributions of investigatory sniffing and hippocampal RSA both shifted with age toward the lower end of their normal ranges. In the youngest animals, the dominant frequency of sniffing matched that of hippocampal RSA (frequency entrainment) more often than would be predicted by chance; preferred phase differences between sniffing and hippocampal RSA were reliably observed in the 5-9 Hz range; and these preferred phase differences varied linearly as a function of frequency, implying an underlying latency relationship. These correlations changed progressively with age as follows: the incidence of frequency entrainments decreased; the frequency range within which preferred phase differences were observed became lower and narrower; and the incidence of preferred phase differences decreased. However, animals of all ages exhibited similar preferred phase differences for those frequencies at which significant preferences were expressed. These findings are discussed in relation to the hypothesis that alterations of forebrain theta rhythms may accompany aberrations of the medial septum-diagonal band-nucleus basalis complex and may be importantly involved in aging-related impairments of cognitive and learning abilities.