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Department of Zoology, University of Hull.

1. In situ measurements of plasma, cerebrospinal fluid (CSF) and brain interstitial fluid (ISF) K+ concentrations have been made in anaesthetized rats aged between 19 days gestation and 30 days after birth using K+-selective micro-electrodes. 2. Under control conditions, plasma, CSF and ISF [K+] did not vary significantly with age, the over-all mean concentrations being 3.8, 3.1 and 3.2 mM respectively. The CSF and ISF [K+] were significantly lower than plasma [K+] at all ages. 3. It is concluded that young rats, from the age of 19 days gestation, can normally maintain plasma-to-CSF and plasma-to-ISF gradients similar to those in the adult, presumably by action of the blood-CSF and blood-brain barriers. 4. During acute plasma hyperkalaemia there was very little regulation of CSF [K+] at 21 days gestation, but after birth there was a gradual improvement in CSF K+ homeostasis until by 30 days after birth a normal CSF [K+] could be maintained. 4. There was no regulation of ISF [K+] during hyperkalaemia at 21 days gestation, although after birth there was a rapid improvement in ISF K+ homeostasis. 1-day-old neonates were able to maintain a normal forebrain ISF [K+] during hyperkalaemia. 6. It is concluded that the onset of both CSF and ISF K+ homeostasis takes place at around birth in rats. However, K+ homeostasis matures more rapidly in the ISF than in the CSF. This suggests that, at least in the neonatal period, the CSF is not essential for ISF [K+] regulation.

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