It is pointed out that the kernel of the inverse response function delta v(
r,t)/delta p(r',t'), whose existence is guaranteed by the Runge-Gross theor
em, cannot Vanish for t<t'. As such the kernel f(xc), which is the function
al derivative of the time-dependent exchange-correlation potential with res
pect to the density, can be symmetric with respect to an interchange betwee
n t and t' without violating causality. Thus there is no conflict between t
he symmetry requirement of this kernel and causality in time-dependent dens
ity-functional theory.