Sympathetic nerve-mediated vasoconstriction in iris arterioles of mature rats occurs via the activation of alpha(1B)-adrenoceptors alone, while in immature rat iris arterioles, vasoconstriction occurs via activation of both alpha1- and alpha2-adrenoceptors. In mature rats the vast majority of sympathetic varicosities form close neuroeffector junctions. Serial section electron microscopy of 14 d iris arterioles has been used to determine whether restriction in physiological receptor types with age may result from the establishment of these close neuroeffector junctions. Ninety varicosities which lay within 4 microm of arteriolar smooth muscle were followed for their entire length. Varicosities rarely contained dense cored vesicles even after treatment with 5-hydroxydopamine. 47 % of varicosities formed close associations with muscle cells and 88 % formed close associations with muscle cells or melanocytes. Varicosities in bundles were as likely as single varicosities to form close associations with vascular smooth muscle cells, although the distribution of synaptic vesicles in single varicosities did not show the asymmetric accumulation towards the smooth muscle cells seen in the varicosities in bundles which were frequently clustered together. We conclude that restriction of physiological receptor types during development does not appear to correlate with the establishment of close neuroeffector junctions, although changes in presynaptic structures may contribute to the refinement of postsynaptic responses.