The present study examined the effectiveness of mechanical and meaningful production of output on the learning of English relative clauses by Iranian EFL learners. The study involved two groups of learners (N = 36) who completed a pre-test, three activities and a post-test over an 8-week period. The collaborative dialogue of the participants was tape-recorded and transcribed. The results of the tests showed that both groups made significant progress from pre-test to post-test and they did not differ significantly in their gains on the target linguistic forms, suggesting that both contexts contributed to the learning of English relative clauses. Findings from the tape-recorded data, however, indicated that there was a significant relationship between activity type and number of language-related episodes (LREs): the more meaningful activities elicited significantly more LREs compared to the more mechanical activities. Furthermore, there was a significant relationship between the gains of learners who scored below the average on the pre-test and the number of LREs they produced. (Contains 1 figure and 8 tables.)