Secretion of adrenocorticotropic hormone (ACTH) by the anterior pituitary is rhythmic and episodic, as reflected by fluctuations in plasma concentrations of ACTH. The present work was designed to further characterize the patterns of ACTH secretion that occur simultaneously within a 24-hour period in the rat. To accomplish this, blood collection protocols with sampling intervals of 2 min, 15 min, and 4 h were used in awake, chronically cannulated rats. Plasma samples were assayed for immunoreactive ACTH, and resultant data were analyzed for significant pulsatile secretory episodes. We observed three different patterns of ACTH secretion within a 24-hour period. Circadian variation occurred with peak plasma ACTH levels in the early evening. In addition, plasma ACTH exhibited two types of episodic variation: (1) episodic bursts with variable amplitudes that occurred approximately three times per hour which have been referred to as 'micropulses', and (2) prolonged elevations of plasma ACTH that occurred approximately 14 times in 24 h which have been referred to as 'larger ultradian' secretory episodes. These latter episodes appeared to consist of groups of relatively high amplitude micropulses. The physiological significance, functional interactions, and location of the controlling oscillator(s) of these different rhythms remain to be determined.