Rats were treated for 24, 48 or 72 h with slow release (SR) emulsions of morphine (75, 100 and 150 mg/kg), cyclazocine (75, 100 and 150 mg/kg) or pentazocine (100, 200 and 400 mg/kg). At these times the degree of physical dependence was assessed by examining the abstinence behavior (jumps + wet shakes), changes in body temperature and body weight induced by naloxone (5 mg/kg). The effects of SR treatments on brain levels of noradrenaline (NA), dopamine (DA), homovanillic acid (HVA), 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) were also determined at these times. The results show that all three opiates induce physical dependence in the order of severity of morphine greater than cyclazocine greater than pentazocine. An elevation of 5-HT turnover also appears to be associated with the dependence produced by these opiates. These findings indicate that the increase in brain 5-HT metabolism is not a primary causative factor during opiate dependence, but occurs in response to some other process.