Cerebrospinal fluid (CSF) amino acid concentrations were measured in six Bipolar I, eight Bipolar II, eight Unipolar, and four other and control patients. All but four were also studied after administration of probenecid. Fourteen amino acids showed significant correlations of concentrations with age of subjects. Significant diagnostic group differences were found for five amino acids; only that of tyrosine persisted after taking subject's age into account. Following probenecid administration, there were statistically significant changes in CSF concentration of several amino acids, but these changes were small and likely indicative of diurnal changes.