Lateral or medial fornix suction ablations were performed on adult male rats in order to selectively ablate or leave intact, respectively, fibers which terminate in the region of the suprachiasmatic nucleus and hypophysiotropic area of the hypothalamus. Plasma adrenal corticosteroid secretion, locomotor activity, body temperature, and food and water intake were recorded at 4 h intervals over a period of 48 h in individual animals 7-10 days postoperatively. Lateral fornix ablation specifically disrupted adrenal corticosteroid periodicity. A least-squares spectrum analysis of the data indicated that corticosteroid may be under ultradian control after this lesion. All animals, regardless of treatment, exhibited normal circadian locomotor activity patterns. Aberrations in feeding, drinking and body temperature rhythms were occasionally observed. This represents a temporary dissociation between the rhythmic expression of corticosteroid secretion and activity, temperature, feeding and drinking. The evidence presented lends support to the multi-oscillator theory of circadian organization, and suggests that the anteroventral subiculum, via the medial corticohypothalamic tract, is important in the regulation of some, but not all, circadian parameters. In addition to the observations on the rhythmicity of locomotor activity, the extent to which the animals are active is also significantly different between groups; ie., the hyperactivity of fornix-transected animals previously reported by others was found to be associated with lateral and not medial fornix ablation.