After occlusion of a fraction of the receptors with the irreversible blocker dibenamine, dissociation constants (K A) of several muscarinic stimulants like carbachol, muscarine, bethanechol and oxotremorine are determined in four types of irides (presence or absence of pigment and/or plasma atropinesterase). Thus, the pharmacological activity of many muscarinic stimulants like carbachol, oxotremorine, bethanechol and muscarine in irides is not influenced by the presence or absence of plasma atropinesterase or the melanin pigment. The dissociation constants obtained from the pharmacological (K A) and biochemical (K D) studies indicates that unequal effects of certain drugs in irides may be largely related to the drug binding to the pigment. Hence, the contribution of the atypical muscarinic receptor, if any, to explain the unequal pharmacological effects in different types of irides must be small.