Interaction between spontaneous and neurally-mediated regulation of tone in the corpus cavernosum smooth muscle (CCSM) of the rabbit was investigated. Changes in isometric muscle tension, intracellular concentration of Ca²⁺ ([Ca²⁺]_i) and membrane potential were recorded. CCSM developed spontaneous contractions, transient increases in [Ca²⁺]_i (Ca transients) and depolarizations. This spontaneous activity was abolished by blocking L-type Ca channels (nicardipine 1 µM), sarcoplasmic reticulum Ca pump activity (cyclopiazonic acid 10 µM), Ca-activated Cl channels (niflumic acid 10 µM) or cyclooxygenase-2 (COX-2; NS-398 1 µM). Transmural nerve stimulation initiated either -adrenergic contractions or nitrergic relaxations of CCSM depending on the level of muscle tone. NS-398 suppressed nerve-evoked contractions by about 70 % but caused only a 40 % reduction in the corresponding Ca transient. Blocking nitric oxide synthase with N-nitro-L-arginine (LNA 100 µM) reinforced nerve-evoked Ca transients by about 150 %, whilst increasing the corresponding Ca transients by only 20 %. In CCSM preparations which had been pre-contracted with either noradrenaline (0.3 µM) or prostaglandin F2 (0.1 µM), nerve stimulation inhibited about 70% of the contraction and caused only a 20 % decrease in [Ca²⁺]_i. Fluorescent immunohistochemistry with COX-2 antibodies and the reverse transcriptase (RT)-polymerase chain reaction (PCR) method showed that the enzyme and its mRNA were highly expressed in the CCSM. These results suggest that spontaneously produced prostaglandins (PGs) not only contribute to the generation of spontaneous contractions but also facilitate nerve-evoked contractions. Conversely, spontaneously released nitric oxide (NO) suppresses excitation. Thus, interaction between spontaneous and neurally-mediated regulation of CCSM tone may be fundamental to maintaining the muscle contractility. In addition, both PGs and NO appear to alter CCSM tone with only small changes in [Ca²⁺]_i.