In this study, the isometric properties and the capillarity of isolated plantaris muscle in hyperthyroid and hypothyroid rats and the relation between these parameters have been investigated. The experiment was carried out in 3 groups as control, hyperthyroidism and hypothyroidism. Each group comprised of 20 Wistar albino adult female rats. In anaesthetized rats, single and tetanic stimulations were applied directly to the plantaris muscle in an isolated tissue bath. From the recordings obtained by an isometric transducer and a polygraph, contraction time, half relaxation time, twitch tension and tetanic force were measured. The plantaris muscle of the other leg was prepared in paraffin blocks after fixation in neutral formalin. 5 mm sections taken from the blocks were stained by haematoxylin and eosin and their capillary densities were determined under light microscope. No significant difference was found in the contraction time between the hyperthyroid and the hypothyroid groups. Half relaxation time decreased in the hyperthyroid group while it increased in the hypothyroid group. Twitch tension and tetanic force decreased in the hyperthyroid group and increased in the hypothyroid group. No difference was found in the capillary density of plantaris muscle between the groups. There was also a lack of correlation between the capillary density of the muscle and the twitch tension and the tetanic force of the muscle in both the hyperthyroid and the hypothyroid groups.