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A 37-year-old woman presented to the emergency department with severe headache, which quickly progressed to altered mental status and seizure activity in hospital. Her initial serum sodium concentration ([Na+]) was 126 mmol/L. In the 24 hours before admission, she exercised vigorously for 120 minutes (interval training plus yoga) and also consumed more than 4 liters of fluid during that time to both stay hydrated and facilitate milk production because she was actively nursing 2 children. Her serum [Na+] and altered mental status corrected slowly over the next 48 hours with furosemide, hypertonic saline, and fluid restriction. This case is unique because it discusses the possible pathogenic role that lactation-induced oxytocin release may have on sustained antidiuresis and dilutional exercise-associated hyponatremia (EAH). This would be the first report documenting EAH in a lactating woman, which may highlight an underrecognized risk factor for physically active women who are concurrently breast-feeding.