Although still at a preliminary stage the original working hypothesis that five distinct prostanoid receptor types exist (3) has been strengthened by studies testing a number of selective agonists and/or antagonists. Such compounds can be systematically used to probe the involvement/presence of prostanoid receptor types in tissues/disease processes to identify the desired profile of new drugs. Although this classification will probably be modified in the light of new pharmacological tools, it seems apparent that the use of such an approach is likely to lead to new therapeutic products in the prostanoid field.