This paper presents evidence which demonstrates that a site within the anterior hypothalamus of the Syrian hamster is essential for the expression of a photoperiodic response but not for the expression of oestrous or circadian activity rhythms. Selective lesions of the anterior hypothalamic nucleus (AHN) were produced by localized infusions of n-methyl aspartic acid (NMA). Histological analysis revealed that the suprachiasmatic and paraventricular nuclei were resistant to the toxin. Circadian locomotor activity rhythms and oestrous cyclicity were unaffected by these lesions but the photoperiodic response of the gonads to short day length (8L:16D) was completely prevented. In marked contrast to control animals, lesioned animals maintained large testes or ovulatory ovaries, even after 10 weeks of exposure to the inhibitory photoperiod. These data suggest that neurones within the AHN form an essential component of the photoperiod time-measuring apparatus which is independent of that controlling circadian rhythmicity.