The open-field activity of first-generation <tex-math>$({\rm F}_{1})$</tex-math> hybrid male C57BL/6 × C3H mice irradiated with γ rays on day 14 of gestation was studied at the following ages: 6-7 months (young), 12-13 months (adult) and 19-20 months (old). Doses were 0.5 Gy or 1.0 Gy. Open-field activity was recorded with a camera. The camera output signal was recorded every second through an A/D converter to a personal computer. The field was divided into 25 <tex-math>$8\text{-}{\rm cm}^{2}$</tex-math> units. All recordings were continuous for 60 min. The walking speed of the 1.0-Gy group recorded at 19-20 months was higher than that for the comparably aged control group. The time which the irradiated group, recorded at 19-20 months, spent in the corner fields was high in comparison with the control group at the same age. Conversely, the time spent by the irradiated group in the middle fields when recorded at 19-20 months was shorter than in the comparably aged control group. No effect of radiation was shown for any of the behaviors observed and recorded at 6-7 and 12-13 months. The results demonstrate that such exposure to γ rays on day 14 of gestation results in behavioral changes which occur at 19-20 months but not at 6-7 or 12-13 months.

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