The duration of migmatite formation in the K and Na geochemical zones was first estimated for the Svecofennides of the northern Ladoga region. The U-Pb (TIMS) age of monazite was determined for the most complete sequence of leucosomes with geological control of their relations and relative age observed in the outcrops. The duration of formation of the leucosomes in two zones is different and is not overlapped by the error of the age analysis: about 5 and no less than 10–15 Ma in the K and Na zones, respectively. The different duration of migmatite formation may reflect different temperature evolution of the rocks of the two zones and also the influence of the composition and rheological properties of migmatizing sequences, which govern the conditions of liquation, transportation, and crystallization of the anatectic melt.