1. The effects of ranges of concentrations of oxyhaemoglobin (0.01-30 microM) and hydroxocobalamin (1-100 microM) were compared for their abilities to reduce relaxant responses to EDRF released by acetylcholine in endothelium-intact rat aortic rings, the nitergic transmitter in rat anococcygeus muscles, and NO in aqueous solution in both tissues (aortic rings were denuded of endothelium). 2. The concentrations of oxyhaemoglobin producing 50% reduction of responses to EDRF and NO in rat aorta correspond closely, the IC50 values being 0.13 +/- 0.02 microM and 0.11 +/- 0.02 microM respectively. 3. Oxyhaemoglobin was equally effective in inhibiting responses to NO in anococcygeus muscles and in aortic rings with an IC50 of 0.14 +/- 0.05 microM. However, responses to the nitrergic transmitter were considerably less sensitive to inhibition by oxyhaemoglobin, the IC50 being 19.7 +/- 5.1 microM. 4. The IC50 values for hydroxocobalamin in inhibiting responses to EDRF and NO in aorta were 3.4 +/- 0.2 microM and 8.4 +/- 0.63 microM, respectively, but it was less effective against responses to NO in anococcygeus muscles the IC50 being 46 +/- 9.6 microM. However, even in the highest concentration used (100 microM), it did not reduce responses to the nitrergic transmitter. 5. The findings are compatible with the views that EDRF is NO, but suggest that the nitergic transmitter in the rat anococcygeus muscle does not behave like free NO.