This study has been carried out to find out the reasons for the wide variations observed in the correlation between serum and saliva lithium levels. Serum/saliva lithium levels were monitored in 10 individuals on 6 occasions. The correlation coefficient (r) varied within individuals from 0.19 to 0.91 (p less than 0.025) whereas when all the individuals were considered simultaneously, it was 0.59 (p less than 0.005). It was concluded that prediction of serum lithium levels from salivary lithium levels would only be possible in those individuals showing good correlation. The variability in this correlation could be mainly due to inter-individual variations in the particular sample of individuals studied.