The relationship between circling behavior and the dopaminergic (DA) innervation of the striatum was investigated in rats. Vmax for DA uptake in crude mitochondrial (P2) fractions was used as a measure of the density of striatal DA terminals. Females, as a group, rotated away from (i.e., contralateral to) the side containing the higher Vmax for DA uptake, while there was a nonsignificant trend in the opposite direction for the males. Further analysis suggested that in both sexes there are two kinds, or populations, of rats: those with their turning biases directed away from (Contra greater than Ipsi rats), and those with their turning biases directed towards (Ipsi greater than Contra rats) the side containing the striatum with the higher Vmax for DA uptake. Evidence supporting this two-population hypothesis includes: (a) For both groups of rats the slope of the best fit linear relationship between the contralateral/ipsilateral Vmax asymmetry and rotational behavior is equal in magnitude, though opposite in sign; (b) Mean contralateral Vmax is greater for the Contra greater than Ipsi rats than for the Ipsi greater than Contra rats, while the mean ipsilateral Vmax is virtually identical for the two groups; (c) The two groups of rats can be differentiated behaviorally on the basis of a measure of total lateralized activity, % turning. In addition, the Km for DA uptake for the females (1.81 +/- 0.07 X 10(-7) M) was found to be significantly greater than for the males (1.51 +/- 0.04 X 10(-7) M; P less than 0.005).