Alterations in behavior are frequently described in rats subjected to portacaval shunt. Previous work has reported reduced spontaneous motor activity in various settings (nighttime, red light, decreased illumination) in this animal model. We investigated this phenomenon in rats of both genders subjected to portacaval shunt to determine whether our previously observed divergent growth patterns (males reduced, females unchanged) had any impact on the alterations in spontaneous motor activity in this model. Dietary intake, growth, motor activity and serum ammonia and amino acid concentrations were measured, in addition to final liver and spleen weights, in each animal after 3 to 4 wk of observation. Our results reconfirm the differential impact of portacaval shunt on growth in male (35 reduction p < 0.01) but not female rats (5 reduction, NS) compared with their respective-gender sham-operated controls. In addition, spontaneous motor activity was significantly reduced in male (≃ 50, p = 0.01) but not female rats subjected to portacaval shunt. The reduction of activity in male rats subjected to portacaval shunt did not correlate with any of the measured biochemical data or calculated nutritional/growth parameters. Thus we observed gender-dependent reduction in spontaneous motor activity after portacaval shunt in the rat. The mechanism for this phenomenon is unknown, but it is easily investigated with this reproducible model. (Hepatology 1994;19:381–388).