Its sources are near Poggio Croce di Prata and Poggio di Montieri in the range of hills known as the Colline Metallifere. From here it takes a north-easterly direction for the first half of its course before turning sharply to the south-east. At Pontiella it receives from the right the waters of the Farma, a torrent born at Torniella, again in the Colline Metallifere). After a few kilometres the Merse enters the Ombrone at Pian di Rocca which lies half-way between the spa of Bagni di Petriolo to the west and Castiglione del Bosco to the east.