The stargazer rat is an autosomal recessive mutant (homozygous stg/stg) that displays abnormal behavior, including profound hyperactivity. Heterozygous stg/+ littermates are unaffected (i.e., are nonmutants), and display normal spontaneous behaviors. Abnormal spontaneous behavior in the stargazer rats suggest that they may be more responsive than their normal littermates to external stimuli and more likely to display anxiety-related behavior in tests of emotionality. To test these hypotheses, the reactive behavior of stargazers and unaffected littermates were compared with regard to their responsiveness to nociceptive stimuli (tail flick test), in the open field test, the elevated plus-maze, and in the swim test of Porsolt. In the open field test, the stargazers spent a greater percentage of the observation period in the open area (p < 0.05), and demonstrated a significantly higher level of locomotor activity (p < 0.05). In the elevated plus-maze, stargazers spent a significantly greater percentage of their total time in the open arms (p < 0.05), but the number of open-arm entries as a percentage of total entries into either arm was not different among the two groups. The stargazers were unable to complete the Porsolt test, perhaps owing to their being ineffective swimmers. No differences among the groups were observed with the tail flick test. Contrary to the original hypothesis, these data suggest that the mutant rats demonstrated less anxiety-related behavior than their nonmutants siblings in tests of emotionality.