Lithium is used in the treatment and prophylaxis of manic-depressive illness. A narrow therapeutic range of lithium (0.4-1.2 meq/l) requires constant monitoring of these levels to avoid neurotoxicity and overdose. It is general practice to measure plasma lithium levels as a guide to monitor the therapy. Efforts to predict the concentration of Li at its active sites in the brain have led to the use of red blood cells (RBCs) because they are similar to neuronal cells. Thus RBC lithium is a very relevant clinical parameter for monitoring therapy and to observe the changes at intracellular levels under varying treatment conditions. A measure of both plasma and RBC lithium may be of significant value to physicians and researchers as lithium profile for RBCs correlate more closely with the brain lithium than plasma lithium. Although methods to measure lithium in blood such as atomic absorption or flame photometry exist, a complete quantitation of both plasma and RBC lithium requires a tedious physical separation of the two components prior to chemical analysis. On the other hand, lithium MR technique, via the use of shift reagents, can provide both plasma and RBC lithium in a single study. Here we have performed a correlation study of lithium results obtained from MR with the vitros dry-slide method on blood samples from rats treated with lithium. The results show a high degree of correlation between the two methods. Additionally, the MR measurements made on dilute samples of blood indicate that small blood samples with lithium concentration in the neighborhood of 0.08 meq/l can be measured with high accuracy and reproducibility needed for clinical purposes.