[123I]5‐I‐A‐85380, which selectively binds to α4β2 nicotinic acetylcholine receptors, may be a suitable single photon emission computed tomography (SPECT) ligand for imaging nicotinic receptors in the human brain. In a mutagenicity study, 5‐I‐A‐85380 produced positive effects in one of five bacterial strains in the bacterial reverse mutation assay only with metabolic activation. The ED50 for 5‐I‐A‐85380 to elicit tonic‐clonic convulsions in mice, 7.1 µmol/kg, i.v., was five times that of nicotine (ED50=1.4 µmol/kg, i.v.). A 2‐day acute toxicity study (0.03–150 nmol/kg, i.v.) and a 14‐day chronic toxicity study (0.3–30 nmol/kg, s.c.) in mice revealed no evidence of gross pathology or histopathological changes. Biodistribution and dosimetry studies revealed that the urinary bladder wall and the thyroid gland were the critical organs with respect to radiation exposure. In unanesthetized rats, 5‐I‐A‐85380 (5.2–174 nmol/kg, i.v.) and nicotine (20–400 nmol/kg, i.v.) produced similar increases in systolic and diastolic blood pressure, heart rate, and locomotor activity. Doses of 5‐I‐A‐85380 that elevated blood pressure and heart rate by 10% ranged from 5–10 nmol/kg, i.v. The highest doses tested, which produced near maximal increases in blood pressure and heart rate, did not affect the PR, QRS, or QTc intervals of the electrocardiogram. Based on these preclinical data, initial SPECT imaging studies in humans (Fujita et al. [2002] Eur J Nucl Med 29:183–190) used a mass dose of 8.6 pmol/kg [123I]5‐I‐A‐8380, estimated to be 1/580th of the dose that elevates blood pressure and heart rate by 10%. Importantly, no clinical pharmacological effects were observed. Drug Dev. Res. 58:149–168, 2003. Published 2003 Wiley‐Liss, Inc.