This article deals with basic determinants of synaptic efficacy during development of glutamatergic and GABAergic synaptic transmission: location and number of release sites, release probability and single cell-activated (unitary) conductances. We hypothesize that both types of neuronal connections differ in major aspects of synaptogenesis. Disregarding the fact that various test models and cell types could render diverging results, it can be observed that glutamatergic terminals display a preference for dendrites, whereas GABAergic terminals select soma locations at initial stages of development. Glutamatergic synapses are characterised by receptor accumulation in the region of terminal apposition, whereas in GABAergic synapses receptor concentration is weak, if present at all. The expression of glutamate receptors (GluRs), but not GABAA receptors is under control of interneurons. Developmental changes in glutamatergic synaptic transmission have not yet been assessed by quantal analysis. For GABAergic synapses, first results are now available from a culture preparation of the rat superior colliculus. In general terms, functional maturation seemed to lag behind the formation of structurally differentiated release sites. Compound binomial analysis revealed that during in vitro development a considerable fraction of GABAergic terminals remained in a low efficacy release state (p < 0.2). A developmental increase in synaptic strength was reached by the appearance of singular highly effective release sites. Presynaptic maturation could be manipulated by long-term drug treatment. Addition of GluR antagonists significantly increased amplitudes and decreased the coefficients of variations of evoked inhibitory postsynaptic currents. Thus, the strength of inhibitory synaptic transmission could be influenced by the status of heteronymous synaptic input.