After 3 tests for male sex behavior, adult male guinea pigs were classified as either low-activity (LA, no ejaculations) or high-activity (HA, ejaculation during at least 2 behavior tests). In 1 experiment, resting levels of peripheral plasma testosterone (T), measured by radioimmunoassay, did not differ between the groups (LA = 2.22 .+-. 0.17 ng/ml, HA = 2.09 .+-. 0.11 ng/ml, .hivin.X .+-. SEM [standard error of the mean]). However, plasma T levels were significantly higher in HA males 2 min after a sex test (2.27 .+-. 0.24 ng/ml) than in LA males (1.40 .+-. 0.20 ng/ml). A 2nd experiment confirmed these results and demonstrated that exposure of HA or LA males to an estrous female placed on the opposite side of a wire mesh barrier similarly led to higher T levels in HA than in LA males. Behavioral differences between HA and LA guinea pigs may not be attributable to differences in resting T levels;) HA and LA males may perceive the sex test situation differently, leading to slight increase in T in HA males and slight decreases in T in LA males, and these changes in T level are probably not dependent on copulation but can be induced by mere exposure to the sight, smell and/or sound of an estrous female.