To investigate the cellular mechanisms underlying the negative force-frequency relationship (FFR) in the ventricle of the varanid lizard, Varanus exanthematicus, we measured sarcomere and cell shortening, intracellular Ca(2+) ([Ca(2+)]i), action potentials (APs) and K(+) currents in isolated ventricular myocytes. Experiments were conducted between 0.2-1.0 Hz, which spans the physiological range of in vivo heart rates at 20-22 degrees C for this species. As stimulation frequency increased, diastolic length, percent change in sarcomere length and relaxation time all decreased significantly. Shortening velocity was unaffected. These changes corresponded to a faster rate of rise of [Ca(2+)]i, a decrease in [Ca(2+)]i transient amplitude, and a seven-fold increase in diastolic [Ca(2+)]i. The time constant for the decay of the Ca(2+)transient (tau) decreased at higher frequencies, indicating a frequency dependent acceleration of relaxation (FDAR), but then reached a plateau at moderate frequencies and did not change above 0.5 Hz. The rate of rise of the AP was unaffected, but the AP duration (APD) decreased with increasing frequency. Peak depolarization tended to decrease, but was only significant at 1.0 Hz. The decrease in APD was not due to frequency-dependent changes in the delayed inward rectifier (IKr) or the transient outward (Ito) current as neither appeared to be present in varanid ventricular myocytes. Our results suggest that a negative FFR relationship in varanid lizard ventricle is caused by decreased amplitude of the Ca(2+) transient coupled with an increase in diastolic Ca(2+), which leads to incomplete relaxation between beats at high frequencies. This coincides with shortened action potential duration (APD) at higher frequencies. Key words: action potential, calcium, diastolic Ca2+, IK1.