Using Monte Carlo simulation and analytical description we studied the variation of catalyst drop composition during III-V nanowire formation via the vapor-liquid-solid growth mechanism. During a layer-by-layer growth mode, a cyclic dependence of arsenic concentration on the growth time in a metal drop was observed. The time shifts between the nucleation of a new layer at the liquid-solid interface and the arsenic concentration maximum in C As(t) and the time shifts between the complete monolayer formation and the arsenic concentration minimum were demonstrated. The reason of these time shifts is the nonmonotonic time dependence of the growing 2D island perimeter.