Eighty Fischer-344 male rats were divided into eight groups. Half the groups were given a high-risk diet (HRD) that was high in fat, low in fiber and low in calcium, while the remaining groups were given a low-risk diet (LRD) that was low in fat, high in fiber and high in calcium. After 4 weeks, four groups were then given two weekly s.c. injections of azoxymethane (AOM) (15 mg/kg body wt), and four groups were given saline injections. At the end of 6 weeks the rats were either continued on HRD, LRD, or crossed over from HRD to LRD, or LRD to HRD for an additional 6 weeks. The rats were then killed and the influence of different dietary regimens on the induction of foci of aberrant crypts (AC) in their colons was compared. The number of foci of AC was highest in the AOM-injected HRD/HRD dietary group (8.83 +/- 0.98), followed by the AOM-injected LRD/HRD group (5.37 +/- 0.75), the HRD/LRD group (3.32 +/- 0.36) and the LRD/LRD group (2.93 +/- 0.32). Except for the HRD/HRD control group, which developed a small but significant number of foci of AC (1.24 +/- 0.19), the other saline-injected dietary groups showed no significant numbers of foci of AC. Two groups of rats on HRD/HRD (AOM and saline-injected), and two groups of rats on HRD/LRD (AOM and saline-injected) were continued on HRD and LRD respectively for an additional 14 weeks. A continuous HRD in AOM-injected rats induced colon tumors in 92% of rats, while the crossover from HRD to LRD suppressed the tumor incidence to 33%. Neither diet induced any tumors in saline-injected rats. The results clearly show that the influence of HRD and LRD on the induction of foci of AC and their distribution in the colon, correlated with the induction and distribution of tumors in the colon.