Calcium desensitisation is defined by the ability of a mediator to cause smooth muscle relaxation independent of a reduction in intracellular [Ca²⁺]. One such mediator is cGMP and cGMP-dependent protein kinase G (PKG). Several protein kinases have been implicated in the above process, and the present study was aimed at investigating the role of protein kinase C and Rho Kinase in calcium desensitisation induced in rat arteries by 8-bromo-cGMP (8-Br-cGMP), a cGMP analogue. Small intrapulmonary arteries (IPA, internal diameter ~200-500 μm) were mounted on a wire myograph, and permeabilised with 60μg/ml alpha toxin in pCa 6.5. IPA were bathed in PIPES-buffered solution (pH 7.1), gassed with 100% air and incubated at 26°C. In all preparations 1μM thapsigargin was present to prevent involvement of intracellular Ca²⁺ stores. Ca²⁺ concentration was regulated by adjusting the ratio of K₂EGTA to CaEGTA. Values are given as mean ± SEM; tests for significance were performed on -log concentrations, using Student’s unpaired t test. The relaxation induced by 8-Br-cGMP (10μM) was found to be Ca²⁺ dependent, being 60 ± 6% at pCa 6.7 (n=4) and 38 ± 12% at pCa 6.4 (n=5), with an IC₅₀ for 8-Br-cGMP of ~25nM. At a pCa 6.4 the Rho kinase inhibitor Y-27632 (10μM) if anything increased relaxation to 8-Br-cGMP (45 ± 13%, n=6), whereas the PKC inhibitor Ro-31-8220 decreased it (18 ± 8%, n=5), though neither reached significance. At a pCa of 6.7, however, Ro-31-8220 caused a significant inhibition of relaxation to 10 μM 8-Br-cGMP (12 ± 13%, n = 7, p<0.01). Due to the degree of depression of tension at pCa 6.7 by Y-27632, its effect on 8-Br-cGMP could not be evaluated, so further experiments were performed on Ca²⁺ response curves. 8-Br-cGMP (100nM) caused a significant rightward shift of the Ca²⁺ response curve (EC₅₀ Control: 157 ± 23 nM; 8-Br-cGMP: 252 ± 47 nM; p=<0.001; n=14). Y-27632 (10μM) alone caused a significant rightward shift of the Ca²⁺ response curve (204 ± 26nM, p<0.05, n=8), but in the presence of Y-27632, 8-Br-cGMP caused a further significant shift to the right (8-Br-cGMP+Y-27632: 306 ± 45nM, p<0.001 vs. Y-27632 alone, n=6). Ro-31-8220 alone also shifted the curve to the right (224 ± 5nM), but in the presence of Ro-31-8220 8-Br-cGMP had no further effect (8-Br-cGMP+Ro-31-8220: 285 ± 18nM (n=4). These results suggest a role for PKC but not Rho kinase in 8-Br-cGMP-induced Ca²⁺ desensitization in rat IPA.