We study genetic variation in phenotypic plasticity maintained by a balance between mutation and
weak stabilizing selection. We consider linear reaction norms allowing for spatial and/or temporal
variation in the environments of development and selection. We show that the overall genetic
variation maintained does not depend on whether the trait is plastic or not. The genetic variances
in height and slope of a linear reaction norm, and their covariance, are predicted to decrease with
the variation in the environment. Non-pleiotropic loci influencing either height or slope are
expected to decrease the genetic variance in slope relative to that in height. Decrease in the ratio of
genetic variance in slope to genetic variance in height with increasing variation in the environment
presents a test for the presence of loci that only influence the slope, and not the height. We use
data on Drosophila to test the theory. In seven of eight pair-wise comparisons genetic variation in
reaction norm is higher in a less variable environment than in a more variable environment, which
is in accord with the model's predictions.