The effects of cadmium on adrenocorticotropin hormone (ACTH) secretion are controversial and seem to depend on the dose and duration of the exposure to the metal. This work was undertaken to analyze the effects of acute cadmium administration on the episodic pattern of ACTH release in adult male rats. For this purpose, animals were cannulated 40 h before the experiment to allow a continuous blood withdrawal. Two and a half hours after the administration of a single dose of cadmium chloride (4.5 mg kg-1 bodyweight), the episodic pattern of ACTH was analyzed during three hours (from 10:30 to 13:30, samples being collected every seven minutes) in conscious and freely moving adult male rats. The mean values of ACTH during the bleeding period and the absolute pulse amplitude were decreased by acute cadmium chloride administration (P < or = 0.001, P < or = 0.01, respectively). By contrast, the frequency of ACTH pulses increased (P < or = 0.01). However, no changes in any other parameters of episodic ACTH secretion were observed compared with control animals. These data suggest that cadmium interferes with the regulatory mechanism of ACTH.