Steroidal neuromuscular blocking drugs (NMBDs) produce a range of effects on the cardiovascular system corresponding both bradycardia and tachycardia. Logical targets, include the muscarinic receptor in the heart (M2) and/or modulation of noradrenergic (NAdr) transmission in this tissue. In an attempt to address these two possible "side effect target sites" this thesis represents a detailed and systematic examination of the effects of a range of steroidal neuromuscular blocking drugs on recombinant human muscarinic receptors (assessed in [3H]N-methyl scopolamine (NMS) binding studies using Chinese hamster ovary cells transfected with M1-M5 muscarinic receptors and cAMP/[Ca2+]i measurements) and noradrenaline uptake-release mechanisms in simple cellular systems (using [3H]NAdr in SH-SY5Y neuroblastoma cells and HEK293 cells expressing uptakei). Four main NMBDs have been chosen; pancuronium (an agent that produces a tachycardia), vecuronium (an agent that products a bradycardia), rocuronium (an agent with variable effects) and gallamine, a well documented muscarinic antagonist (an internal positive control). [3H]-NMS binding was displaced in a concentration dependent manner from m1-m5 receptors by all NMBDs. Further kinetic studies with pancuronium, vecuronium and gallamine indicated an allosteric action at M2 receptors. Methacholine inhibition of cAMP formation at the M2 receptor was reversed by pancuronium. Rocuronium and vecuronium produced a small but significant shift in the methacholine response. At clinically achievable concentrations no NMBD affected [Ca2+]i. Collectively these data conclusively identify pancuronium (at clinical concentrations) as a moderately selective M2 receptor antagonist with an additional allosteric mode of action. Studies such as these are important to enable clinicians to make rationale evidence based choices in their use of neuromuscular relaxants.