The Finish military has sent letters to 900,000 reservists to collect their contact details and remind them of their duties "in the event of war."
The letters, which were first sent earlier in May, tell former conscripts which regiment or unit to report to in the event of hostilities.
Recipients say it is the first time such letters has been sent for many years.
The government denies that the move is connected to the Ukraine crisis or rising tension with Russia.
In recent months, Russian warplanes have frequently probed Finnish air defenses.
And in April 2015, the Finnish Navy dropped depth charges in waters near the capital, Helsinki, as a warning to a suspected submarine, which some media reports said was Russian.
Finland, which is not a NATO member, shares a 1,340-kilometer border with Russia.