Female rats were tested for behavioural laterality in a T-maze (TM) and a "handling by the tail" (TH) test, and vaginal smears were taken every day. The two tests did not correlate. There was a population bias of left choices in TH, but in TM most of the animals were ambilateral. Statistical analysis showed that animal learning in successive trials influenced their lateral choice, but it was similar in both tests. The difference in lateral bias in rats in the two tests depended mostly on differences in internal laterality. Rats in proestrus had a left-side bias in TM. There was no dependency between direction of lateral choice in TH and estrus cycle.