Metarule phrase-structure grammars (MPS grammars) have been shown to be an extremely powerful formalism in need of constraints from both the computational and the linguistic points of view. One problem with the standard generative interpretation of metarules is the generation of infinite rule sets. Furthermore, even if grammars having this property are disallowed, the possibility of a combinatorial explosion of rules still remains. In the present paper we explore a view of metarules as meta-node-admissibility conditions (MNACs) which allows a non-generative interpretation of metarules. Under such an interpretation, an MPS grammar will not have either of the two problems mentioned above. We find that, under one suggested implementation, the above mentioned problem appears under another guise, so that additional constraints are needed to ensure an effective procedure for checking admissibility conditions in the computational setting. The important observation is that one can, by parsing with MNACs on the fly, recognize languages for which the generative interpretation is not available.