In Section 1 we state the known theory of reproductive equations in general. The key result is that every equation, having at least one solution, is equivalent to some reproductive equation. In Section 2 we extend the notion of reproductive equations to the class of equations $Eq(x)$ which are solved by means of some given equation, denoted by $Eq_1(\varrho)$. In that case we also prove that such an $Eq(x)$, having at least one solution, is equivalent to some reproductive equation.