The effect of several nitric oxide releasing-non-steroidal anti-inflammatory drugs (NO-NSAID) and nitroprednisolone on blood vessel relaxation in vitro and in vivo was studied. Nitroflurbiprofen (NOF; EC(50), 688.8+/-93.8 microM), nitroaspirin (NOA; EC(50), 57.9+/-6.5 microM), nitroparacetamol (NOPARA; EC(50), 71.5+/-14.6 microM) and nitroprednisolone (EC(50), 15.1+/-1.4 microM) caused concentration-related relaxation of noradrenaline (NA)-contracted rat aortic rings. All NO releasing compounds tested were approximately three orders of magnitude less potent than sodium nitroprusside (SNP, EC(50), 35.7+/-3.5 nM). The vasorelaxant effect of NOF and NOPARA in the rat aorta was potentiated by zaprinast (5 microM) and reduced by ODQ (5 microM). Flurbiprofen and paracetamol (100 microM) caused minimal (<10%) relaxation of the rat aorta and did not affect the response to SNP. The effect of NOF was unchanged in the presence of L-NAME (100 microM; EC(30), 181.8+/-35.1 microM cf. EC(30), 125.1+/-17.0 microM, P>0.05) but increased by removal of the endothelium (EC(30), 164.3+/-26.3 microM cf. EC(50), 688.8+/-93.8 microM, P<0.05). NOF (0.1 - 50 microM) produced a small but not concentration-related vasodilation of the NA-preconstricted (i.e. "high tone") perfused rat mesentery preparation (cf. SNP, EC(30), 4.4+/-0.7 microM). In contrast, NOF (1 - 100 microM) produced concentration-related vasodilation of the "high tone" perfused rat kidney with an EC(50) of 33.1+/-4.4 microM. Neither NOF (74 mg kg(-1), i.p.) nor NOA (91.9 mg kg(-1), i.p.) nor equimolar doses of flurbiprofen (50 mg kg(-1), i.p.) or aspirin (50 mg kg(-1), i.p.) affected mean arterial blood pressure (MAP) or heart rate (HR) of pentobarbitone-anaesthetized rats over a 1 h period. NO-NSAID relax blood vessels in vitro by an NO-dependent mechanism. The absolute vasorelaxant effect of NO releasing drug varies greatly with the choice of compound and between blood vessel preparations.