Summary Sustained use of non-steroidal anti-inflammatory drugs (NSAIDs) may prevent colorectal cancer. However, the optimal drug, period of efficacy and mechanism(s) of action are unknown. Experiments were undertaken to determine which of several NSAIDs would modulate colon crypt cell proliferation or apoptosis when given during the initiation phase of 1,2-dimethylhydrazine (DMH)-induced rat colon cancer. Colon crypts located both away from and over an aggregate of lymphoid nodules (ALN) were examined. Rats were injected with aspirin, indomethacin, nabumetone, sodium salicylate, 16,16-dimethyl prostaglandin E2 or saline for 3 days and DMH or DMH vehicle on day 4 of each week for 8 weeks, then killed 3 days after the last DMH injection. At the time of killing, DMH had significantly increased crypt cell proliferation but not apoptosis. There was significantly more cell proliferation and apoptosis in crypts over the ALN than away from the ALN. Aspirin and salicylate increased proliferation and apoptosis in crypts over the ALN. Finally, the distributional peaks of cell proliferation and apoptosis were shifted significantly closer together after DMH. Thus, DMH increases proliferation and alters the distribution of proliferating and apoptotic cells in colon crypts early in carcinogenesis. Aspirin may suppress tumour incidence via salicylate by enhancing apoptosis in carcinogen-initiated cells.Keywords: colon cancer; proliferation; apoptosis; non-steroidal anti-inflammatory drugs; lymphoid nodules; 1,2-dimethylhydrazine Data from human epidemiological research (Thun, 1994;Giovannucci et al, 1995) and from rodent experimental research (Craven and DeRubertis, 1992;Reddy et al, 1993) have demonstrated both correlative and direct evidence of a role for aspirin and possibly other non-steroidal anti-inflammatory drugs (NSAIDs) in the prevention of colon and rectal cancer morbidity and mortality. However, the optimal NSAID, the dose, the period of efficacy and the mechanism of this putative chemopreventive activity remain to be established (Earnest et al, 1992;Giardiello et al, 1995).One potential mechanism for this chemopreventive effect lies in the ability of aspirin and other NSAIDs to inhibit prostaglandin production by blocking the activity of one or both of the prostaglandin endoperoxide G/H synthase (PGHS) isozymes, PGHS-1 and PGHS-2 (Meade et al, 1993). PGHS-l is the constitutive form of the enzyme and is expressed in most tissues, whereas PGHS-2 is induced by mitogenic stimuli in inflammatory situations (Smith and DeWitt, 1996) and is expressed at high levels in colon tumour tissue (Eberhart et al, 1994;Kargman et al, 1995;Dubois et al, 1996). Aspirin covalently and irreversibly modifies the cyclo-oxygenase active site of both PGHS-1 and PGHS-2 by acetylation, whereas other NSAIDs reversibly block PGHS function and thus prostaglandin production via steric hindrance of the cyclo-oxygenase active site (Smith and DeWitt, 1996 (Marnett, 1992). These include: evidence that PGs stimulate proliferation of human colon can...