1. Sodium hydrogen sulphide (NaHS), a donor of hydrogen sulphide (H(2)S), produced dose-related relaxation of the rabbit isolated ileum (EC(50), 76.4+/-7.9 microM) and rat vas deferens (EC(50), 64.8+/-5.4 microM) and reduced ACh-mediated contraction of the guinea-pig isolated ileum. 2. NaHS also reduced the response of the guinea-pig (EC(50), 80.0+/-5.7 microM) and rat (EC(50), 108.2+/-11.2 microM) ileum preparations to electrical stimulation of the intramural nerves. In guinea-pig ileum this effect was spontaneously reversible and mimicked by sodium nitroprusside (SNP, EC(50), 2.1 microM). Combination of NaHS (20 microM) with SNP (0.5 microM) produced a greater than additive inhibition of the twitch response of the ileum to electrical stimulation. 3. The inhibitory effect of NaHS on the field-stimulated guinea-pig ileum was unaffected by pretreatment with L-NAME (100 microM), indomethacin (10 microM), naloxone (1 microM) or glibenclamide (100 microM). Furthermore, NaHS (200 microM) did not affect the contractile response of the ileum to KCl (10 to 60 mM). 4. Propargylglycine (PAG, 1 mM) and beta-cyanoalanine (BCA, 1 mM) (inhibitors of cystathionine-gamma-lyase) but not aminooxyacetic acid (AOAA, 1 mM) (inhibitor of cystathionine-beta-synthetase) caused a slowly developing increase in the contraction of the guinea-pig ileum to field stimulation. This effect was reversed by cysteine (1 mM). 5. These results show that NaHS relaxes gastrointestinal and urogenital smooth muscle and suggest that H(2)S is responsible for these effects. The possibility that endogenous H(2)S, formed as a consequence of activation of intramural nerves, plays a part in controlling the contractility of the guinea-pig ileum is discussed.