['To investigate whether the relaxation of the striated urethral sphincter in guinea pigs is mediated by nitric oxide.', 'After sacrifice, urethral rings were cut between the bladder neck and the penile crura and mounted in an in vitro bath. Maximal isometric tension obtained at 1, 3, 10 and 25 Hz electrical stimulation (Tmax) for 5 to 30 sec was measured.', 'Tmax at 25 Hz was not changed by pretreatment with 10(-4)M nitroprusside or 10(-4)M NG-nitro-L-arginine. In urethral tissue precontracted with 10(-4)M noradrenaline, a significant relaxation of 16-25% was found at 10 Hz. This relaxation could not be prevented by pretreatment with 10(-4)M NG-nitro-L-arginine and was not enhanced by the presence of 10(-3)M L-arginine.'] Is urethral relaxation after electrostimulation in the guinea pig independent of nitric oxide?