Two studies on measurement of M capacity are reported. Study 1, with 191 subjects aged 6-11, found factor-analytical and correlational evidence that five M capacity tests share a common source of variance, and that, with age, they increase at a similar rate. Study 2, with 124 subjects aged 6-10 years, replicated the previous findings. It is suggested that, in this age range, M capacity can be measured with a battery of tests.