The parasitic worm Ascaris suum contains the opiate alkaloid morphine as determined by HPLC coupled to electrochemical detection and by GC-MS. The level of this material is 1168 ± 278 ng/gm worm wet weight. Furthermore, Ascaris maintained for 5 days contained a significant amount of morphine, as did their medium, demonstrating their ability to synthesize the opiate alkaloid. To determine if the morphine was active, we exposed human monocytes to the material and they immediately released nitric oxide in a naloxone-reversible manner. The anatomic distribution of morphine-immunoreactivity reveals the material is in the subcuticle layers and in the animals' nerve chords. Furthermore, as determined by RT-PCR, Ascaris does not express the transcript of the neuronal mu receptor. Failure to demonstrate the expression of this opioid receptor, as well as the morphine-like tissue localization in Ascaris, suggests that the endogenous morphine is intended for secretion into the microenvironment.