‘Chemically skinned’ muscle fibre bundles of pig carotid arteries and natural actomyosin from the arterial media were compared. In both cases the concentration of Ca<sup>++</sup> ions required for half maximal tension development and 50% activation of the Mg++-activated ATPase was similar and strongly pH dependent (10<sup>–4</sup> m at pH 6; 10<sup>–6</sup> m at pH 7). The optimum pH for contraction and Mg++-activated ATPase activity of glycerinated fibre bundles was found to be near neutrality, while it was below six in the case of A dilute natural actomyosin. This discrepancy seemed to be due to the fact that at pH 6 the actomyosin was insoluble. Neutralization in the presence of Mg<sup>+</sup>+-ATP solubilized natural arterial actomyosin even at low ionic strength. This solubilization inhibited the apparently gel dependent actomyosin ATPase activity. This inhibition could be reversed if the gel formation was produced by increasing the protein concentration: the activity rose to the higher level characteristic for actomyosin in the gel state within glycerinated fibres.