The antimicrobial effect of methenamine in urine is dependent upon: (i) the rate of generation of formaldehyde; (ii) the concentration of formaldehyde achieved; and (iii) the duration of bacterial exposure to formaldehyde. Studies utilizing an in vitro model which simulates the dynamics of the urinary tract showed that bacteriostatic concentrations of formaldehyde (greater than or equal to 25 mug per ml) were obtained from attainable concentrations of methenamine at urine pH 5.7 to 5.85. Reduced urinary flow rates and large residual volumes increased formaldehyde concentrations and would appear to provide prolonged bacterial exposure to the formaldehyde.