Two beta-adrenoceptor agonists, clenbuterol and ketoclenbuterol, were examined for their effects on growth and cardiac tissue. In female rats, clenbuterol caused a 48% increase in weight gain (P < .05), with improved feed efficiency (26%; P < .1) and increased muscle mass (9%; P < .1). Ketoclenbuterol had less effect on weight gain (30%) and feed efficiency (16%) and did not increase muscle mass. Next we studied the adverse cardiovascular effects of these compounds. Neither drug increased the force of contraction of isolated rat ventricular papillary muscle. Clenbuterol was potent at causing an increase in the rate of contraction of isolated rat atria, and when fed to cattle over 2 d, the drug caused heart rate to increase by 92 to 117%. In contrast, ketoclenbuterol was not a potent stimulator of atrial rate in the rat, and in cattle it caused a smaller increase in heart rate than clenbuterol (12 to 27%). Finally, cattle that were underfed to simulate dry-season tropical pasture conditions were treated with clenbuterol or ketoclenbuterol for 35 d. Ketoclenbuterol caused no beneficial changes in N metabolism. The results obtained with clenbuterol were equivocal, and might have been confounded partly by the refusal of some treated animals to eat all the feed offered. Although clenbuterol did not cause a reduction in total urinary N output relative to control animals, marked reductions in plasma urea concentrations and in urea synthesis were observed (23 to 53%; P < .001). We conclude that ketoclenbuterol is not effective for attenuation of dry-season protein loss in cattle. Clenbuterol seems to be less effective in underfed cattle than in well-fed cattle, and further evidence is required to judge whether compounds of this nature are likely to benefit tropical cattle under harsh grazing conditions.