A histological, physiological and pharmacological study of two regions of the isolated uterine muscle of the rat., i.e., the myometrial one (a zone close to the mesometrial insertion) and the linea uteri (the antimesometrial zone) was performed. From the histological point of view the linea uteri (LU) is characterized by the presence of small bundles of longitudinal smooth muscle fibers (20 to 50), closely packed and clearly surrounded by a connective tissue, whereas in the myometrial zone (M) the smooth muscle fibers are less closely packed and the connective tissue is surrounding a greater number of them. The spontaneous oxytocin or electrically-induced initial tension developed by the LU or the M, was similar. Also the stability with time did not differ between both regions. Atropine depressed consistently the isometric developed tension of M strips, either spontaneously active or driven by oxytocin or electrical stimulation, but had no action on the mechanical activity of preparations from the LU. A pretreatment with hemicholinium also inhibited the spontaneous as well the oxytocin-induced motility of M strips, without affecting those of LU. Pharmacologic evidences suggest that acetylcholine may play a "facilitating" role in M strips for the conduction of coordinate contractions. This does not appear to be the case in the LU, which has the required characteristics to be recognized as a conductive like tissue.