text: The affinity of an antagonist for its binding site (K), i.e. its ability to bind to a receptor, will determine the duration of inhibition of agonist activity. The affinity of an antagonist can be determined experimentally using Schild regression or for competitive antagonists in radioligand binding studies using the Cheng-Prusoff equation. Schild regression can be used to determine the nature of antagonism as beginning either competitive or non-competitive and K determination is independent of the affinity, efficacy or concentration of the agonist used. However, it is important that equilibrium has been reached. The effects of receptor desensitization on reaching equilibrium must also be taken into account. The affinity constant of antagonists exhibiting two or more effects, such as in competitive neuromuscular-blocking agents that also block ion channels as well as antagonising agonist binding, cannot be analyzed using Schild regression. Schild regression involves comparing the change in the dose ratio, the ratio of the EC of an agonist alone compared to the EC in the presence of a competitive antagonist as determined on a dose response curve. Altering the amount of antagonist used in the assay can alter the dose ratio. In Schild regression, a plot is made of the log (dose ratio-1) versus the log concentration of antagonist for a range of antagonist concentrations. The affinity or K is where the line cuts the x-axis on the regression plot. Whereas, with Schild regression, antagonist concentration is varied in experiments used to derive K values from the Cheng-Prusoff equation, agonist concentrations are varied. Affinity for competitive agonists and antagonists is related by the Cheng-Prusoff factor used to calculate the K (affinity constant for an antagonist) from the shift in IC that occurs during competitive inhibition. The Cheng-Prusoff factor takes into account the effect of altering agonist concentration and agonist affinity for the receptor on inhibition produced by competitive antagonists.