The results are given from monitoring a commercial closed‐formula cereal‐based rodent diet (Purina 5010), two open‐formula cereal‐based diets (NIH‐31 and NIH‐07), and one purified diet (AIN‐76) for selected nutrients and contaminants. The observed concentrations of nutrients (protein, fat, vitamin A, and thiamine) approximated the manufacturer specifications for closed‐formula cereal diet, while the average concentrations of nutrients found in the open‐formula cereal diets were well above the nominal concentrations. Nominal concentrations for these open‐formula diets tended to be close to the minimum values that were observed. Except for protein levels, greater variation in nutrient concentrations was found in the purified diet than in the cereal diets. Contaminants were generally much lower in the purified diet than in the cereal diets, but the variation of contaminants was about equal in the two types of diets. Open‐ and closed‐formula cereal diets appear to be very similar to each other in the degree of variation of nutrients and contaminants. Cadmium, lead, and selenium are the constituents of greatest concern in assuring the quality of the rodent diets that were evaluated.