The tonus and rate of spontaneous contractions measured in vivo from the tunica albuginea of rabbit testes were augmented by epinephrine, acetylcholine and prostaglandin (PG)F2 α. Neither reserpine, dibenamine nor atropine decreased autorhythmic contractility. Indomethacin injections ip significantly decreased the mean amplitude of testicular contractions. This treatment effaced motility in two of eleven preparations. Injections of 5,8,11,14-eicosatetraynoic acid (TYA) did not significantly decrease the amplitude of contraction. Electrical stimulation of the spermatic artery (site of the spermatic nerve) in rabbits and in rats failed to alter testicular tonus or motility. The above evidence suggests that local factors such as PGs may contribute more to smooth muscle activity in the tunica albuginea of rabbit testes than do autonomic nerves. The rhythmical contractions observed in vivo, however, continued in the presence of indomethacin in the bathing medium, and may not require stimulation by PGs for their origination. Rat testes, unlike rabbit testes, did not contract rhythmically in vivo. Isolated rat testes contracted in response to administered PGF2 α, while PGE1 abolished the induced contraction. Such modulators of muscular tonus feasibly could alter intratesticular pressure for the intact animal.