The purpose of the article is presentation of studies that firstly: were a replication of Wegner's and Erber's (1992) studies which were related with paradoxes of controlling thoughts (suppression and concentration); secondly: enriched it by the stream-of-thought stage. Our studies were an extended version of Wegner's and Erber's experiment, where the participants (in conditions of time pressure or lack of it) gave their associations to the words they had hear. Participants' tendency to associating words they hear with the word the instruction to suppress (Experiment 1) or to concentrate on (Experiment 2) were related to was investigated in that way. In the last stage participants related their stream of awareness marking out appearance of thought about the key object. In the time pressure condition we obtained results that testify paradoxical effects of the effort to suppress thoughts. The concentration task, however, did not lead to the mental control paradox. The obtained results argue that the paradoxical effects of mental control occur in attempts to suppress thoughts more easily than in attempts to concentrate. In case of the concentration task the mechanism is competitive to the paradoxical processes; the mechanism is a greater cognitive accessibility of the content that the instruction to concentrate on was referred to.