The effects of pirenzepine and atropine on basal pancreatic exocrine secretion were studied in conscious rats fitted with a chronic pancreatic fistula. Pirenzepine was about 50 times less potent than atropine on a molar basis in inhibiting pancreatic secretion (respectively 44x for volume and 51x for protein output), while the efficacies of both drugs were in the same range. These results indicate that basal pancreatic secretion in vivo is mainly regulated by muscarinic mechanisms poorly sensitive to pirenzepine, and analogous to the M2 receptors described by others in vitro.