Summary.The times children take to reach correct solutions to seven easy Block Design items from the try‐out version of the British Intelligence Scale were analysed in terms of two regression models. The times were obtained from 395 children aged between 10 and 13 years, each of whom had a score of four or more on the full 16‐item test. The simpler of the two models was found to be the most satisfactory, the model specifying that the time to correct solution of a person on a test item is a proportional function of the difficulty of the item and a single parameter describing the speed of the person. Individual speed parameters were estimated for each child and were analysed in terms of an age X ability analysis of variance. This analysis indicated highly significant main effects, speed increasing with both age and ability. There was no interaction between age and ability. Results were related to previous findings and supported a view of mental speed as a major component of abil