Potassium-contracted guinea pig taenia coli relaxed when exposed to hypoxic glucose-free conditions, and this was followed by a second contraction. During this second contraction, which was not inhibited by ethylene glycol-his-(beta-aminoethylether)-N,N'-tetracetic acid (EGTA) the muscle was in rigor. After development of this contraction the muscle did not have an active state and was less extensible than a muscle normally contracted with KCl. Tension during rigor in an isometrically contracting muscle was short-lived in contrast to shortening during rigor development in a lightly loaded isotonically contracting muscle. These findings, together with the observation of irreversible lengthening of a muscle in rigor on stretching, suggest that rigor linkages can be broken. Glucose removal under aerobic conditions did not produce rigor, but oxygen removal in the presence of glucose resulted in relaxation followed by secondary contraction. This later contraction, unlike that accompanying rigor, was reversed by removal of calcium with EGTA. These data imply that in the absence of oxygen the removal of cytoplasmic free-Ca2+ is inhibited.