Two experiments are described, one of which demonstrated that the frequency and length of blocks in mental work are related to the amount of conflict between the elements in the task, while the other showed that when such conflict is absent the frequency and length of the blocks are relative to the homogeneity of the task. An additional experiment was carried out in order to see how the blocking in the manual performance of a task compares with the blocking in the vocal performance of that task, and whether the blocking developed in one performance is carried over partially or completely to the other performance. The results pointed to a close relation between the two types of performance and suggested that the same process is involved in both. Further analysis disagreed in part with this conclusion. (PsycINFO Database Record (c) 2006 APA, all rights reserved)