The release of substance P (SP)-like immunoreactivity (SP-LI) in the dorsal horn of the spinal cord and the cardiovascular changes to both high-tension (HT) and low-tension (LT) contractions were determined using α-chloralose-anesthetized cats. Over a 10-minute period, seven contractions (HT or LT) were induced. Each contraction was 20 seconds in duration and was followed by an 80-second quiescent period. The tension-time index (TTI) for the HT contractions was 2751±348 kgs (mean±SD), which was greater than the TTI of 813±167 kgs for the LT contractions. The HT contractions caused a greater release of SP-LI than the LT contractions: SP-LI increased from 0.18±0.02 to 0.32±0.03 fmol/100 μL and from 0.18±0.02 to 0.25±0.04 fmol/100 μL for the two types of contractions, respectively. Concomitant with this greater SP-LI release, HT contractions caused larger increases in mean arterial pressure (34 ±16 versus 11±4 mm Hg) and heart rate (18±7 versus 8±4 beats per minute) than did the LT contractions. These changes in SP-LI, mean arterial pressure, and heart rate were virtually abolished when the contractions were repeated after sectioning the L-5-S-2 dorsal and ventral roots or when the electrical stimulation of the ventral roots was repeated after muscle paralysis with gallamine triethiodide. These results demonstrate that contraction-evoked SP-LI release in the dorsal horn is related to the developed tension. Furthermore, these data provide additional support for the hypothesis that the release of SP from the central terminations of muscle afferents plays a role in mediating the cardiovascular responses to static contraction of skeletal muscle.