  The nonperturbative aspects of string theory are explored for non-critical
string in two distinct formulations: loop equations and matrix models. The
effects corresponding to D-brane in these formulations are especially
investigated in detail. It is shown that matrix models can universally yield a
definite value of the chemical potential for an instanton while loop equations
can not. This implies that string theory may not be nonperturbatively
formulated solely in terms of closed strings.
