The purpose of this study was to characterize hepatic cytochrome P450 induction in the dog by phenobarbital, beta-naphthoflavone, dexamethasone, and isoniazid using catalytic activities and Western blots with antibodies prepared against rat cytochrome P450 isozymes. Male beagle dogs were treated with phenobarbital (10 mg/kg for 2 days and 30 mg/kg for the following 5 days), beta-naphthoflavone (50 mg/kg for 5 days), or isoniazid (10 mg/kg for 2 days and 30 mg/kg for the following 5 days). Female beagle dogs were treated with dexamethasone (50 mg/kg for 5 days). Increases in the liver/body weight ratio were observed after treatment of dogs with phenobarbital (133% of control) and dexamethasone (153%). Total cytochrome P450 content was increased as a percentage of control after treatment with phenobarbital (264%) and (3-naphthoflavone (186%), while it slightly decreased after treatment with isoniazid (54%) and dexamethasone (71%). Dog liver microsomes hydroxylated testosterone mainly at the 6-beta and 16-alpha positions but also at the 6-alpha-, 15-beta-, 15-alpha-, 16-beta-, 18-, 2-beta-, and 17-positions. There were no sex differences in terms of regio-selectivity of testosterone metabolism between control male and female dogs. Treatment of dogs with phenobarbital produced increases in 6-beta- (184%), 16-alpha- (379%), 16-beta- (210%), 18- (195%), and 2-beta-testosterone (203%) hydroxylase and pentoxyresorufin 0-dealkylase (651%) activities. On Western blots, phenobarbital treatment produced induction of P450 3A- and 2B1-related proteins. Although treatment with dexamethasone resulted in a large increase in liver weight, no significant increase in P450 3A-related protein or 6-beta-hydroxylase activity was detected. However, dexamethasone and isoniazid treatment produced slight increases in chlorzoxazone hydroxylase activity. Treatment with isoniazid induced a P450 2E1-related protein. Treatment with (beta-naphthoflavone produced increases that were 689 and 357% of control in ethoxyresorufin 0-deethylase and chlorzoxazone hydroxylase activities, respectively. Beta-Naphthoflavone treatment increased the amount of two proteins immunochemically related to the cytochrome P450 1A subfamily. Thus, although generally similar to other species, the response of the dog to cytochrome P450 inducers differs significantly from the rat and human in some cases.