The effects of capture and temperature stresses on plasma concentrations of testosterone (T) and progesterone (P) were examined in wild male tuatara (Sphenodon punctatus), the only living sphenodontid reptile. Free-roaming male tuatara on Stephens Island, New Zealand, had low levels (0.3–0.5 ng/ml) of plasma T in July (winter), and high levels (15–17 ng/ml) in January (summer). Levels in November appeared intermediate (5 ng/ml) but were measured with reference to standards prepared in human rather than tuatara plasma. These seasonal variations are consistent with available information on the annual cycle of spermatogenesis and reproductive behavior in male tuatara. In November, plasma T was unchanged in males bled at 0, 3, and 12 h after capture but had fallen significantly to 57% of the initial value by 24 h after capture (experiment 1). In January, males held in collecting bags for 3 h prior to blood sampling had levels of plasma T similar to those of free-roaming males sampled at the same time (experiment 2). In July, an acute heat stress (12.8°C elevation in body temperature over 30 min) had no effect on plasma T (experiment 3). Significant changes in plasma P were seen prior to changes in plasma T in experiment 1: plasma P was elevated at 3, 12, and 24 h after capture. No difference in plasma P was observed between free-roaming and captive males in experiment 2. In experiment 3, no change in plasma P occurred in response to heat stress. Free-roaming males showed no evidence of a diurnal cycle in plasma T or plasma P in November. These data indicate that plasma T in male tuatara is unaffected by short-term (3 h) capture; however, longer-term (24 h) capture combined with repeated blood sampling causes a significant reduction in plasma T, as previously reported for other male reptiles. Changes in plasma P suggest that activity of the adrenal gland may be sensitive to captivity stress in this species. These data provide the first values for circulating steroid levels in free-roaming wild male tuatara and help define conditions of captivity that influence plasma steroid concentrations, a prerequisite for further studies of steroid cycles in this species.