Five possibilities of defining a coefficient of facilitation and inhibition are described. It is shown that the application of these definitions to the same spike train activity eventually leads to considerably different results, e.g., a response which is inhibitory according to one definition sometimes is facilitatory according to another definition. To find the most reliable definition the theoretical differences between the five alternatives are examined, whereby a coefficient is considered reliable if it is reproducible and independent of external experimental parameters, such as the record length. As an experimental example spike trains were recorded from mitral cells in the olfactory bulb of the goldfish. We divide every response into two sections: an initial reaction and a steady state reaction. In this way each response can be uniquely classified. The most reliable definition of a coefficient of facilitation turns out to be based on the steady state levels of an averaged peristimulus time histogramme. Under certain conditions this corresponds to considering the means of the sample mean rates of a stochastic point process before and after the stimulus application, whereby the initial reactions are neglected. These should be classified by other methods and notions.