Conscious dogs chronically implanted with a device for cerebrospinal fluid (CSF) sampling from the anterior 3rd ventricle were submitted to 24 h dehydration. During rehydration by drinking the total water intake ( TWI ) after 16 min was determined in 8 and after 90 min in 14 experiments. Samples were simultaneously drawn to determine the osmolalities (Posm, CSFosm ) and AVP concentrations (PAVP, CSFavp ) of plasma and CSF. After 24 h dehydration all of these parameters were significantly elevated in comparison to euhydrated dogs investigated on 19 occasions. In 8 experiments 60% of the final TWI had been ingested within the first 16 min with no changes of Posm, CSFosm and CSFAVP , but a significant decrease of PAVP at this time. TWI per kg body weight ( TWI X kg-1) after 90 min was significantly correlated with the osmolalities and AVP levels in plasma and CSF prior to rehydration. The decreases of Posm, CSFosm and PAVP, but not of CSFAVP , were significantly correlated with TWI X kg-1. The results indicate that PAVP and CSFAVP are subject to long term control by body fluid tonicity exhibiting a feedback relationship to water intake. In addition, PAVP but not CSFAVP seems to be under short term, possibly nonosmotic, control during water intake.