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 one 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, X +/- SEM). 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 second experiment confirmed these results and also 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. These results indicate that a) behavioral differences between HA and LA guinea pigs are not attributable to differences in resting T levels, b) HA and LA males perceive the sex test situation differently, leading to slight increases in T in HA males and slight decreases in T in LA males, and c) these changes in T level are not dependent on copulation but can be induced by mere exposure to the sight smell and/or sound of an estrous female.