According to a recent report, mood cycles in a group of patients with rapid cycling bipolar disorder oscillated in synchrony with lunar gravimetric tides. Mood switches in a 67-year-old woman with rapid cycling bipolar II disorder on lithium maintenance treatment were assessed with a χ periodogram and a χ analysis of the mood switches in relation to the lunar tidal cycle. During a period when she was treated with nortriptyline and her thyroid-stimulating hormone levels were elevated, her mood switches had a significant (P<0.05) 29- to 30-day periodicity, and the χ analysis showed that the switches were distributed nonrandomly in relation to the spring-neap lunar tidal cycle (P<0.0001); 14 of 15 switches occurred within 2 days of the spring tides. After nortriptyline was discontinued, thyroid-stimulating hormone levels were normalized with treatment with levothyroxine, and consistent bright light treatment was started, the synchrony between mood cycles and lunar cycles disappeared, and rapid cycling eventually stopped. The possibility that lunar mood cycling is sometimes contingent on antidepressant treatment, decreased thyroid function, and certain types of light-dark cycles needs to be considered in future research on lunar tidal influences on the course of bipolar illness.