Sources: Coffee, tea, cola nuts, mate, guarana.
     Effects: Stimulant of central nervous system, cardiac muscle,
	    and respiratory system, diuretic Delays fatigue.

     Sources: Tea
     Effects: Cariac stimulant, smooth muscle relaxant, diuretic, 

     Sources: Cocoa bean (1.5-3%) Cola nuts and tea
     Effects: Diuretic, smooth muscle relaxant, cardiac stimulant, 

The presence of the other alkaloids in colas and tea may explain 
why these sometimes have a stronger kick than coffee. Colas, which
have lower caffeine contents than coffee are, reportedly, sometimes
more active. Tea seems the strongest for some. Coffee seems more 
lasting for mental alertness and offers fewer jitters than the 

Info from Merck Index
Reference Variability in caffeine consumption from coffee and
tea: Possible significance for epidemiological studies by B.
Stavric, R. Klassen, B. Watkinson, K. Karpinski, R. Stapley, and
 P. Fried in "foundations of chemical toxicology", Volume 26,
number 2, pp. 111-118, 1988 and an easy to read overview, Looking
for the Perfect Brew by S. Eisenberg, "science news", Volume 133,
April 16, 1988, pp. 252-253.

Quote from the lab manual:

  Caffeine is present in tea leaves and in coffee to the
  extent of about 4%. Tea also contains two other
  alkaloids, theobromine and theophylline. These last two
  relax the smooth muscles where caffeine stimulates the
  heart and respiratory systems.

The effects of theobromine are, compared to caffeine and
theophylline, relatively moderate. However, cocoa contains eight
times more theophylline than caffeine. As well, caffeine has been
shown to combine with other substances for added potency. Thus
the effects of theobromine might be enhanced by the caffeine in

On humans caffeine acts particularly on the brain and skeletal
muscles while theophylline targets heart, bronchia, and kidneys.

Theobromine is highly toxic to dogs and kills many canids/year
via chocolate poisoning. It takes quite a dose to reach fatal
levels (more than 200 mg/kg bodyweight).  Reference: Fraser,
Clarence M.,et al, eds. The Merck Veterinary Manual, 7th
ed. Rahway, NJ: Merck & Co., Inc. 1991. pp. 1643-44.