Few studies have reported the effects of gastrin-releasing peptide (GRP)/bombesin on the guinea pig gallbladder, and the results are contradictory. Because such contradictory results may, in part, be due to technical factors, we investigated the effect of GRP on guinea pig gallbladder smooth muscle, using an improved horizontal organ bath. The guinea pigs were killed and the gallbladder was removed. Four longitudinal muscle strips (2 x 12 mm) were suspended in Krebs-Ringer solution at 37 degrees C and aerated with 95% O2 and 5% CO2. The mechanical activity of the strips was recorded isotonically by displacement-voltage transducers, via L-arms, to which a piezoelectric element with a frequency of 100 Hz and movement of 50 microns was applied. GRP contracted gallbladder muscle strips dose dependently, but the calculated maximal response was 22.4% and 20.1% of the acetylcholine- and cholecystokinin octapeptide (CCK8)-induced responses, respectively. The GRP-induced contraction was unaffected by the muscarinic blocker, atropine, or by the CCK receptor antagonist, loxiglumide. It is concluded that GRP weakly, but apparently directly, stimulates guinea pig gallbladder contraction.