On-line processing data were obtained to examine an inverse relationship between judged grammaticality of sentences and reaction times (RTs) needed to judge their grammaticality. 13 students read the initial portion (context string) of sentences and then judged whether their final portion (target string) continued grammatically or ungrammatically with the context string. Types of violation used involved a lexical category rule, selectional restriction on a phrase boundary and selectional restriction on a noun phrase. Analyses showed a predicted inverse relationship, i.e., violation of lexical category was detected most quickly and rated more ungrammatical than the other two types of rule violation. No difference was found between the latter two types of violation. The results extend Moore's (1972) finding for on-line judgments of grammaticality of sentences. Findings were discussed in relation to the concept of grammaticality employed in previous studies.