The main objective of this study was to provide a description of the possible patterns of rat behaviour in the exploration box. Both age and sex of the animals were controlled.The measures taken assessed changes in the patterns of behavioural activity over time. Sex did not differentiate the rat's behaviour significantly.Two patterns of developmental changes were found. The results of cluster analysis with a sequential data sets lead to the conclusion that there are at least two levels of regulation of rat behaviour: activation and content organization.It was also shown that rat behaviour in the exploration box is not stereotyped but highly plastic. The results are discussed in terms of development, sex differences and individual differences.The Study of exploratory behaviour in the rat has impHcations for both the study of individual differences (Henderson, 1994;Pisula, Ostaszewski, Osiriski, Trojan & Matysiak, 1995) and the development of psychological theory in general (Matysiak, 1992;Renner, 1990).There are several unsolved methodological issues in the study of exploratory behaviour. One is the selection of appropriate data analysis techniques.Another is the use of ecologically meaningful testing apparatus. The utility of multivariate analysis has been demonstrated in open-field behaviour (Pisula, 1994) and investigatory responses (Renner & Seltzer, 1994). It seems to be most useful in identifying behavioural strategies, particularly sequential analysis of behavioural transitions (Pisula, 1994; Renner & Seltzer, 1994). Thus it seems to indicate that the use of a priori indices are of little use. The use of multivariate techniques involving both quantitative and qualitative measures is more fruitful.A second methodological issue concerns the appropriateness of the open field test.