Two alpha 1-adrenoceptor subtypes (alpha 1A and alpha 1B) have been detected in various tissues by pharmacological techniques, and three distinct cDNAs encoding alpha 1-adrenoceptor subtypes have been cloned. The profile of an increasing number of subtype-selective compounds at cloned and endogenous receptors recently has facilitated alignment between cloned and pharmacologically defined alpha 1-adrenoceptor subtypes. Thus, alpha 1a-adrenoceptors (previously designated alpha 1c), alpha 1b-adrenoceptors and alpha 1d-adrenoceptors (previously designated alpha 1a, alpha 1d or alpha 1a/d) are now recognized. Since the alpha 1d-adrenoceptor shares characteristics with both alpha 1A- and alpha 1B-adrenoceptors, tissues previously reported to express alpha 1A- and/or alpha 1B-adrenoceptors may additionally contain alpha 1d-adrenoceptors. This article reviews the features of all three subtypes and discusses possible pitfalls in their pharmacological identification.