Elucidation of the electrophysiological characteristics of uterine smooth muscle is important for understanding the electrical events that regulate parturition at the end of pregnancy [1]. Similarly, this may be informative in developing tocolytic drugs directed against ion channel targets for the treatment of premature labour. Two steps are important in this regard. First, it would be informative to determine the spatiotemporal characteristics of action potential (AP) propagation in the uterus in order to identify how electrical excitation-contraction coupling may be co-ordinated at a tissue and organ level. Second, consideration should be given to possible off-site actions of drugs targeting electrogenic events. In this regard, it is important to also understand the actions of electrogenic modulators on cardiac AP waveforms. The guinea pig is a useful model of human cardiac [2] and uterine [3] function. We, therefore, have undertaken pilot experiments using high frequency optical mapping to examine the spatiotemporal characteristics of AP propagation in hearts and uteri isolated from adult female guinea pigs. Fluorescence was excited by light-emitting diodes (at 488 nm) and the filtered emission images (590 nm long pass) were acquired by a high speed camera (up to 2.5 kHz) with a viewing area of 128×128 pixels, resulting in spatial resolutions of 200 x 200 μm2. Following well-established methods [4], the optical mapping system was validated by recording epifluorescence images from perfused ex vivo intact hearts loaded with voltage-sensitive dye di-4-ANEPPS and mechanical uncoupler blebbistatin (Fig 1). Uterine tissues were mounted on a silicone support and superfused with MOPS-based Krebs solution at 37 °C. After a period of stabilization, the tissues were loaded with di-4-ANEPPS (5-10 μM), contraction was minimised with wortmannin (17-25 μM) and the fluorescent images were acquired at 0.5 kHz (Fig 2). In summary, our optical mapping procedure enables recordings of up to 0.4 msec and 200 μm resolution in 2.5 x 2.5 cm2 areas of isolated hearts and uteri from guinea pigs. To our knowledge, these are the first reported visualisations of AP spatiotemporal patterns in uterine smooth muscle and offer much promise to add to our knowledge gathered thus far from spatially distributed surface electrode recordings [5]. In addition, the ability to visualise AP characteristics in both uterine and cardiac tissues will assist with (i) elucidating tissue-specific mechanisms of electrogenesis and (ii) investigating putative tissue-specific actions of ion channel modulators in both tissue types from one animal.