This paper presents the results of a bidirectional selective breeding experiment involving 1,287 mice. It was conducted by means of ~-ambulation-that is, by summing positive and negative leamingassociated alterations in open-field ambulation over a 5-day period of training. ~-ambulation remained constant, but a selection response was found in maze running times and error scores over generations and sexes in the two diverging, intralinearly homogenous, strains of mice.In this paper, we report the first bidirectional selective breeding experiment by means of Ll-ambulation (i.e., the summed average day-to-day acquisition-associated positive and negative alterations in open-field [OF] ambulation) conducted and administered on two lines of mice. This experiment served to illuminate the complex relationship between OF activity and spatial learning. Previously, the acquisition of a new stimulating task has been thought to be positively reflected in enhanced, measurable, overt movements in phylogenetically low organisms, such as rodents (K. Y. H. Lagerspetz, Tirri, & K. M. 1. Lagerspetz, 1968). Thus, only increased motor activation pertaining to learning was, as a rule, reported. For example, Mitani, Ando, and Nagata (1972) trained white rats on a foodreinforced runway. The experienced rats enhanced their activity level in a running wheel relative to their untrained counterparts. Furthermore, Milkovic, Paunovic, and Joffe (1976) observed that OF activity was increased by prior avoidance conditioning. The possibility that a new acquisition experience modifies the motor activity bidirectionally was touched on in an experiment concerning the impact of familiarity with the environment when interindividual variations were noted in learning-induced ambulation (Kvist, 1983 Lagerspetz, 1980). This notion was further supported by an OF-ambulation experiment conducted in conjunction with maze learning and passive-avoidance conditioning on outbred Swiss albino and NMRI mice and on inbred Balb/c, C3H1He, C57BLl6J, and DBAl2N animals. The outbred Swiss albino and NMRI mice reacted similarly to the inbred Balb/c animals, and hybrids ofthese Correspondence should be addressed to S.