Abstract Mentha x piperita L., and five other Mentha species of mint were grown in Northern Finland at the North Ostrobothnia Research Station of the Agricultural Research Center at latitude 64°40â²N and 25°05â²E. To investigate the influence of harvest dates on the oil composition, leaves were collected at an early flowering stage (50% of buds) (I) and when 30% were in full bloom (II). Different harvesting times did not greatly affect the percentage of the main components for any of the mints. At this stage of plant development, young leaves contained more menthone than menthol, and apparently the amount of menthol had only started to increase. Also the relative percentages for menthofuran and pulegone were low in the leaves. The compositional studies were performed by GC/MS on solvent extracts of the leaves.