Axisymmetric laboratory gravity currents are used to study the structure and dynamics of gravity current heads during the inertia-buoyancy phase of the flow. The currents are produced by releasing a fixed volume of heavy fluid behind a gate into less dense ambient fluid. Using particle image velocimetry, vertical and horizontal velocity fields are obtained at different times during the evolution of the flow. During the early stages of the inertial phase, vertical velocity fields reveal a cyclic process of vortex formation. One cycle consists of three parts: formation of a vortex due to baroclinic vorticity at the leading edge of the head, formation of a vortex of opposite circulation along the bottom surface, and convection of heavy fluid forward by the vortex pair. The 'new' heavy fluid at the leading edge initiates a new cycle. Mixing is due not only to small-scale shear instabilities at the interface but also to large-scale entrainment of ambient fluid into the gravity current head as it develops. In horizontal velocity fields, significant azimuthal nonuniformities are observed between the radially expanding vortices and the bottom surface.