To clarify the causes of SFD [small-for-date] [in humans] from the viewpoint of placental function, investigations were made on feto-maternal amino acid metabolism and on how cyclic[c]AMP behaved in the placenta, particularly in relation to the transport of amino acid across the placenta. Amino acids were measured by GLC (H-flame ion detector), cAMP by the binding protein method, adenylate cyclase activity by Krishna''s method and proteins by Lowry''s method. The levels of maternal amino acids were indistinctive between the SFD group and the normal pregnancy group. The feto-maternal [f/m] ratio of amino acid concentrations was lower in the SFD group than in the normal control. The level of cAMP in the placenta was lower in the SFD group than in the control. The lower level of placental cAMP in the SFD placenta seems partly attributable to the decreased adenyl cyclase activity. There was a positive correlation (r [correlation coefficient] = 0.61) between the content of cAMP in the placenta and the f/m ratio of amino acid concentrations. There was a decline in placental active transport of amino acids in SFD; adenylate cyclase and cAMP in the placenta presumably play a key role in such course.