This paper reports on a study designed to investigate psychological factors that affect access to metalinguistic knowledge in second language (L2) production. Based on previous cognitive and psycholinguistic research, it was hypothesized that real-time access to metalinguistic knowledge would be largely determined by three interacting factors: attention to form, processing automaticity, and linguistic prototypicality (i.e., whether a rule concerns a central or peripheral use of a target structure). The subjects were 64 adult Chinese learners of English. A verbalization task was used to assess their metalinguistic knowledge about 12 target uses, and a judgment test was administered to determine the relative prototypicality of these uses. Attention to form was operationalized by two consciousness-raising tasks and by time pressure. Analyses of the subjects' output on writing and error-correction tasks revealed significantly greater grammatical accuracy for more prototypical uses and on tasks that allowed more attention to form. There was also a significant interaction between prototypicality and attention to form, suggesting the influence of processing automaticity. These results are taken as evidence that there are major psychological constraints on the utility of metalinguistic knowledge in L2 performance.