Seasonal and daily variation in basal plasma concentrations of corticosterone and the effect of short-term confinement on plasma corticosterone concentration were examined in wild adult male and female tuatara (Sphenodon punctatus) on Stephens Island, New Zealand. Males and females sampled immediately upon capture at night (active period) showed significant variation in basal plasma corticosterone among four seasons of the year, and females also showed variation in concentration between reproductive states sampled at the same time. Highest basal concentrations in females were seen in November, when the level in gravid females about to nest (4.45 +/- 0.66 ng/ml; mean +/- SE) was almost twice that in nonnesting females at the same time (2.48 +/- 0.30 ng/ml). Plasma samples collected from tuatara at dusk, middle of the night, dawn, and middle of the day showed no significant daily variation in corticosterone concentration in either summer or winter. In these samples, significant positive correlations between body temperature and log plasma corticosterone were observed in males in summer and winter, but not in females in either season. Mean corticosterone concentrations in tuatara held in cloth bags for 3 h were significantly higher than in free-roaming controls (P < 0.01), with mean concentrations in males about 23 times, and vitellogenic females 9 times, those of their respective controls. Plasma corticosterone concentrations measured in this stress study were compared with previously published concentrations for plasma sex steroids in the same individuals. Corticosterone was positively correlated with progesterone (both sexes), but not with testosterone (both sexes) or estradiol (measured in females only). In summary, wild adult tuatara have low basal levels of plasma corticosterone that vary seasonally, but not diurnally. As in other reptiles, corticosterone concentration shows a significant elevation in response to short-term confinement.