Relationships between the binding and the distribution of drugs have been studied in vitro and compared with in vivo data. By use of a standardized technique of distribution dialysis, 10 model drugs were allowed to be distributed among blood and homogenates of seven tissues. The drugs represented a variety of distinct molecules with different lipophilicities, ionization constants, and binding characteristics. The tissue/blood drug concentration ratios were below unity for salicylic acid and phenylbutazone, at about unity for antipyrine (phenazone) and morphine, and above unity for two barbiturates and four basic lipophilic drugs. The binding of the 10 drugs to blood and homogenates of seven tissues was determined by use of conventional equilibrium dialysis and experimental conditions identical to those used in distribution dialysis. From these binding values (free fractions), the theoretical concentration ratios were calculated. There was a good correlation between the calculated values and those determined by distribution dialysis. Thus, the distribution of drugs in the in vitro model of distribution dialysis clearly is the result of binding competition and is predictable from binding values. The correlation between distribution in vitro (or calculated from binding values) and distribution in vivo, on the basis of literature data, indicated a reasonable agreement for antipyrine and the acidic lipophilic drugs used, as well as for the basic lipophilic drugs, with respect to the brain, muscle, and adipose tissue. However, the distribution of the latter drugs in the lungs, liver, and kidneys was grossly underestimated.(ABSTRACT TRUNCATED AT 250 WORDS)