Thirty-six adult male and female Wistar rats were divided into three groups, each of which underwent different training programmes in which greater relevance was attached to the intensity of exercise than to its duration. Samples of the longissimus lumbaris muscle taken between the second and fourth lumbar vertebra, were stained with m-ATPase after acid preincubation at pH 4.4, and with NADH-TR. The most notable results obtained were: (a) none of the training programmes involved gave rise to changes in the ratio of type I: type II fibres; (b) the proportion of IIA fibres increased in all cases, while the proportion of IIB fibres decreased correspondingly; (c) the percentage of FT fibres decreased significantly, and in greater proportion than the decrease recorded in IIb fibres, and (d) specific speed training schedules led to greater changes in the fibre composition of this muscle.