o-Iodosobenzoic acid (IBA), in a surfactant micellar medium, is a rapid and efficient catalyst for the hydrolysis of organophosphate (OP) esters. Since little is known about the toxicity of IBA, a primary screen of neurobehavioral toxicity was evaluated in male ICR mice. IBA was administered intraperitoneally in a pH 7.4 phosphate buffer solution containing 8% dimethylformamide. The predominant overt signs of toxicity included an immediate and transient writhing reflex and/or persistent spasmodic myotwitching of the abdomen, and conspicuous suppression of orienting/exploratory behavior and emotional defecation. The dose ranges for ED50 of writhing response, suppression of rearing and spontaneous motor activity overlapped at levels of about one-tenth the acute LD50, 742 (633-856) mumol/kg, being 94.9 (74.5-122.5), 69.8 (47.9-105.4) and 71.1 (49.9-101.3) mumol/kg, respectively; the dose ranges for ED50 of abdominal myotwitching and depression of emotional defecation in a novel environment also overlapped but at levels of about one-fifth the acute LD50, being 138.4 (115.3-167.2) and 146.2 (110.7-196.3) mumol/kg, respectively. Morphine (1.25-10 mg/kg s.c.) antagonized the IBA-induced writhing response and abdominal myotwitching in a dose-dependent manner, with a PD50 of 4.2 and 4.9 mg/kg, respectively. The present report demonstrates that acute intraperitoneal administration of IBA produces an intriguing, non-specific behavioral syndrome, probably resulting from nociceptive stimulation. This implies that IBA might be irritating to the skin and mucosa.