This article reports on a study that investigated whether processing instruction(PI) or production-based instruction (PBI) is more effective for the teaching of regular past simple verb forms in English. In addition, this study examined whether explicit grammatical information (EI) mediates the effectiveness of PI or PBI. A total of 194 Turkish EFL students were randomly assigned to one of the four experimental groups--PI+EI, PI-EI; PBI+EI, PBI-EI--or a control group and then completed interpretation and production tasks. The results demonstrated that (a) the PI-EI group and PBI-EI group performed equally well on both interpretation and production tasks; (b) when EI was a factor, the PI+EI group outperformed the PBI+EI group on only the interpretation task, while no significant difference was found on the production task; (c) no significant differences were found between the PI+EI or -EI groups, and the PBI+EI or -EI groups. Pedagogical implications of these findings are discussed, and suggestions made for future research.