The mechanisms associated with the neurotoxic responses caused by prolonged exposure (48 hr) to norepinephrine (NE) were examined in cultures of brain stem of 18-day-old rat fetuses. Two separate components of NE neurotoxicity were identified and differentiated according to dose dependency, sensitivity to catalase and blockade by adrenoceptor antagonists. The first component of NE toxicity was responsible for the death of the overall cell population, affecting both neurons and astroblasts, and was mediated by NE auto-oxidation products. This toxicity was observed at high doses of NE (LD50: 100 microM), was mimicked by other catecholamines (epinephrine, isoproterenol, dopamine), was fully antagonized by catalase and could not be blocked by adrenoceptor antagonists. The second component of NE toxicity was specifically targeted at noradrenergic neurons and was mediated by alpha 1 adrenoceptors. The specific toxicity for noradrenergic neurons was seen at lower doses of NE (LD50: 20 microM) and epinephrine (LD50: 40 microM). It was mimicked by the alpha 1 agonist phenylephrine and blocked by the alpha antagonists prazosine and nicergoline. These results indicate that protracted exposure to catecholamines may be a possible cause of damage to noradrenergic neurons that can be prevented by alpha 1 adrenoceptor blockade.