OBJECTIVE: The authors hypothesized that changes in brain membrane composition resulting from omega-3 fatty acid administration in patients with bipolar disorder would result in greater membrane fluidity, as detected by reductions in T(2) values. METHOD: Women with bipolar disorder (N=12) received omega-3 fatty acids for 4 weeks. A cohort of bipolar subjects (N=9) and a group without bipolar disorder (N=12) did not receive omega-3 fatty acids. T(2) values were acquired at baseline and after 4 weeks. RESULTS: Bipolar subjects who received omega-3 fatty acids had significant decreases in T(2). There was a dose-dependent effect when the bipolar omega-3 fatty acid group was subdivided into high- and low-dose cohorts. CONCLUSIONS: Omega-3 fatty acids lowered T(2) values, consistent with the hypothesis that the fluidity of cell membranes was altered. Further studies are needed to clarify the significance of alterations in brain physiology induced by omega-3 fatty acids, as reflected in T(2) values.