The action of a phosphodiesterase inhibitor, RGW 29 38, was analysed experimentally. A preliminary study with guinea pig isolated heart, using Langhendorf's method, revealed a cardiotonic effect, though less than with isoprenaline and dobutamine, with which it was compared. The inotropic action of the compound was then studied using human isolated atrium. The model is described. It consisted of measurement by a strain gauge of the contractions obtained in a stimulated atrial fragment obtained during a surgical procedure. The following parameters were analysed, expressed as percentage increase in relation to baseline conditions: maximum tension developed, positive dP/dt and negative dP/dt. The effect of RGW (n = 6) was compared with that of isoprenaline (n = 7). RGW caused an increase in maximum tension of 195% +/- 91%, in positive dp/dt of 110% +/- 72% and in negative dp/dt of 168 +/- 54%. This increase was, however, less than that seen in the isoprenaline group. Thus RGW had a definite positive inotropic effect on guinea pig isolated heart and human isolated atrium, though less than that of the catecholamines with which it was compared. Isolated human atrium appears to be a useful study model, in particular for analysis of the inotropic action of drugs.