An internal combustion engine includes a driving shaft, a pair of camshafts for driving engine valves, a transmission connecting the driving shaft to a first of said camshafts and a transmission connecting the first camshaft to the second camshaft. The transmission connecting the two camshafts to each other includes a pair of articulated parallelogram mechanisms each having two crank members rotatable with end portions of the camshafts and connected to each by means of a connecting rod. The crank members are made up of circular discs eccentrically mounted on the camshafts and rotatably received in circular openings formed at the ends of the respective connecting rod. The two crank members rotatable with the same camshaft are spaced from each other by a determined angle.