The brain concentration of lithium (Li) in treated rats was measured using a recently developed method based on in vivo ⁷Li PRESS localized MRS. Comparison was made to the corresponding serum concentration at two treatment durations. The brain and serum Li concentrations were highly correlated with each other, more so than found previously for humans. The brain and serum Li concentrations also correlated with dose. Both the brain Li concentration and the serum concentration at 16.1 days of treatment correlated with the corresponding measure at 6.6 days. After correction of the brain Li concentrations for reduced ⁷Li MRS visibility, the mean brain/serum Li ratio for rats was close to unity, unlike most previous values found for humans. However, in some individual cases the ratio deviated substantially from the mean, suggesting that serum Li is not always a reliable indicator of brain Li. Magn Reson Med 52:1087–1092, 2004. © 2004 Wiley‐Liss, Inc.