We have previously shown that brief, meal-contingent infusions of the amphibian peptide bombesin (BN) reduce meal size in spontaneously feeding rats. These inhibitory effects presumably reflect actions of BN on receptors for its mammalian homologues, which include gastrin-releasing peptide1-27 (GRP) and neuromedin B-10 (NMB). In the present study, we used the spontaneous feeding paradigm to explore the effects of meal-contingent infusions of these mammalian homologues. Undisturbed, ad lib-fed male rats (n = 12) with chronic inferior vena caval catheters were infused with saline, 5 nmol/kg GRP, 5 nmol/kg NMB, and a mixture of 5 nmol/kg GRP and 5 nmol/kg NMB in counterbalanced order, with intervening noninfusion days. Infusions were remotely activated at the onset of the first nocturnal meal and continued for 3 min (34 microliters/min), delivery being completed within the first 2 min of infusion. Feeding was measured via electronic balances linked to a computer. The effects of all peptide conditions were confined to the first nocturnal meal. Significant and comparable decreases in the size and duration of this meal were observed with GRP, NMB, and GRP + NMB, with no significant effects on the intermeal interval or satiety ratio (intermeal interval/meal size). Thus, brief vena caval infusions of GRP and NMB, given alone or together at the onset of the first nocturnal meal, significantly reduced meal size and duration in spontaneously feeding rats.