A theoretical account is given for long- and short-term changes in populations of neurons subject to independent rules for pre- and postsynaptic modification. The postsynaptic rule proposes that coactivated heterosynaptic inputs to a neuron alter the states of ion channels at a given synapse, thereby changing the susceptibility of these channels to local biochemical alterations. The resultant change in the population distribution of local channel states affects the postsynaptic potential produced at the synapse by subsequent inputs. This postsynaptic rule applies, in general, to short-term changes at specific individual synapses. In contrast, the presynaptic rule applies in general to long-term changes in the whole neuron, resulting in an altered probability of transmitter release. Because of neuroanatomical constraints, the presynaptic rule affects large numbers of synapses defined by the connectivity of that neuron and distributed nonspecifically over the population. We show that the combined action of the two independent rules upon populations of neurons arrayed in interconnected neuronal groups leads to consistent alterations of the probability of firing of certain circuits while maintaining variability in the response of the population to novel input. This "dual rules" model fulfills the requirements of the theory of neuronal group selection.