An automated method for rapidly measuring surface area of individual cardiac myocytes was used as an index of myocyte growth. Hearts from 2-to 4-day-old rats were digested by overnight incubation in cold trypsin solution. Enriched suspensions of myocytes were plated at 2 x 105 ceils/well in 12-well-culture plates. Cells were grown in M199 supplemented with 1%, 10% serum or 10% serum plus 10 -7 M norepinephrine. On days 1-4 after plating, cells were fixed in Bouin's Solution and stained with Weigert's Iron Hematoxylin and Biebrich Scarlet-Acid Fuchsin. An inverted microscope, video camera and monitor were coupled to a video image processor (Image Technology Corp.). The enhanced image of stained heart cells was digitized, and perimeter, length, width and area of each selected cell were calculated. One hundred randomly selected cells were measured in each of eight wells from each treatment-day group. Areas of individual myocytes varied widely in culture dishes and the distribution was skewed toward larger cells. The standard deviation increased in proportion to an increase in mean cell area. A logarithmic transformation of the data normalized the data and yielded a more homogeneous variance. The geometric mean area of heart cells supplemented with 1% serum increased only slightly, but significantly, during four days in culture. Geometric mean area of cells supplemented with 10% serum increased nearly four-fold. Supplementing cells with norepinephrine (10 -7 M) in addition to 10% serum did not induce a further increase in cell size. This technique has the potential to rapidly and objectively monitor heart cell growth following pharmacological or toxicological treatments.