Samples of mice from each of 6 inbred strains were tested for audiogenic and electroconvulsive seizures at 5 ages (14, 21, 28, 35, and 42 days). A moderately large within-strain correlation (.67) was found, indicating that developmental patterns of susceptibility to audiogenic and electroconvulsive seizures are similar within each strain. The finding of an even larger between-strain correlation (.91) indicated that strains which are highly susceptible to audiogenic seizures are also likely to be highly susceptible to electroconvulsive seizures. In a 2nd experiment, whole brain norepinephrine and serotonin were assayed in each of 5 inbred strains at 21 and 42 days of age. Results were consistent with the hypothesis that levels of these amines are inversely related to seizure susceptibility. Mice from strains which were susceptible to seizures at 21 days of age had significantly lower levels of norepinephrine and serotonin in brain than did 42-day-old, seizure-resistant animals.