Lateral ventricular volumes were monitored and quantified using accurately registered magnetic resonance images (MRIs) in six healthy individuals 30 min before and up to 4 h after ingestion of a glucose drink. The volume of the lateral ventricles increased by an average (± S.E.M.) of 2Σ4 ± 0Σ4 % as blood glucose levels rose from 4Σ8 ± 0Σ2 mmol l-1 to 8Σ4 ± 0Σ4 mmol l-1. This was followed by a peak decrease of 5Σ99 ± 3Σ3 % below initial fasting volumes as blood glucose levels fell to 5Σ0 ± 0Σ3 mmol l-1. We suggest that the secondary volume decrease demonstrates a homeostatic process of brain volume regulation for which the mechanism remains uncertain.