The alpha-adrenoceptor blocking agent, tolazoline, and the O-methylated metabolite of adrenaline, metanephrine, produced a concentration-dependent increase of tension in the smooth muscle of the cat isolated nictitating membrane. These effects were considered to be due to the activation of postsynaptic alpha-adrenoceptors. 2 The responses to nerve stimulation of this muscle were neither potentiated nor blocked by tolazoline (0.1 to 10 micrometer) or metanephrine (1 to 10 micrometer). 3 3H-transmitter overflow evoked by electrical stimulation was not modified by tolazoline or metanephrine in concentrations in which these drugs stimulated the postsynaptic alpha-adrenoceptors. 4 Since tolazoline and metanephrine failed to activate the presynaptic alpha-adrenoceptors of the cat nictitating membrane under experimental conditions in which they stimulated the postsynaptic alpha-adrenoceptors, these results further support the view that the presynaptic (alpha2) adrenoceptors differ from the postsynaptic (alpha1) adrenoceptors.