To establish whether adrenal corticosterone secretion is episodic and whether episodes are random or periodic events, adrenal extracellular fluid was sampled in awake rats by intra-adrenal microdialysis. Experiments conducted 1 or 2 days postsurgery between 1000 and 1830 h consisted of continuous collection of dialysate at intervals of 10 min. Pulses detected by PC-Pulsar had an interpulse interval of 54.1 +/- 4.5 and 51.3 +/- 5.0 min on days 1 and 2, respectively. Fourier analysis was used to determine whether significant periodicity was present. Six of seven of the day 1 animals had significant secretory rhythmicity, with a predominant periodicity at 60-80 min. In day 2 animals, in addition to a period near 50-80 min, a lower-frequency rhythm with a period near 213 min was identified. Autocorrelation analysis produced results qualitatively similar to Fourier analysis. Secretion rates were normalized based on maximal adrenal responses to adrenocorticotropic hormone and averaged at each time point. When the composite series was subjected to Fourier analysis, a significant rhythmicity (period 58 min) was still detectable in the day 1 animals. When composite data for the day 2 animals were analyzed, in addition to a component near 60-80 min, a lower frequency component near 320 min was present. The persistence of periodicity after time averaging suggests that corticosterone pulses are synchronized between animals. Cross-correlation analysis of individual pairs of data series supported synchronization in many cases. These results suggest that pulsatile secretion of corticosterone in rats has a primary periodicity near 60 min and is synchronized across animals. The mechanism responsible for this synchronization is unknown.