The main objective of this study is to investigate three equalized batches of a wild Mentha longifolia (ML) population from northern Hungary for extractability by gentle ultrasonic extraction, antioxidant activity, and phenolic constituents. This is the first analytical investigation in a Hungarian population of this species. Antioxidant properties of ML extracts were evaluated with DPPH, photochemiluminescence-O2-. scavenging assay, FRAP, and total polyphenol measurements. HPLC-DAD determination was performed for the main phenolic constituents. The extracts are medium or strong antioxidants in DPPH and FRAP assays and show strong O2-? scavenging capacity. Rutin, quercetin, kaempferol, apigenin, naringenin, rosmarinic, vanillic, gallic, syringic, caffeic, and ferulic acids were identified. All batches were unexpectedly rutin-rich (1700-7500 mg/kg dry herb) and also were rich in rosmarinic acid. Further characteristic traits in the polyphenol profile were a lack of naringenin and (previously not reported) the presence of free kaempferol (up to 30 mg/kg dry herb).