The effects of altering the concentrations of various cerebrospinal fluid (CSF) cations on hypothalamo-hypophyseal-adrenocortical (HHA) activity were investigated in conscious cats. The cerebroventricles were perfused with CSF containing high or low [Na+], [K+], [Li+], [Ca2+] and [Mg2+] for 60 min and plasma samples taken periodically for analysis of cortisol. The results demonstrate that changing CSF [Na+] or [Li+] does not substantially alter plasma cortisol levels. Although individually elevating CSF [K+], [Ca2+] or [Mg2+] does not affect plasma cortisol levels, reducing these CSF cations activates the HHA system and the latter in turn is inhibited by the addition of dexamethasone (DEX) to low cation CSF. The excitatory action of reduced [Ca2+] is also inhibited by the addition of norepinephrine to the perfusion fluid, suggesting that lowering this cation affects the HHA system by preventing the release of the adrenergic neurotransmitter. Furthermore, these data suggest that in conscious unrestrained cats basal HHA activity is dependent to some extent upon normal brain extracellular [K+], [Ca2+] and [Mg2+].